Server Dashboard SSL Support
Added a simple if-else statement that checks TLSOnly mode enabled. If enabled, serves the dashboard in HTTPS mode. Otherwise, serves the dashboard in HTTP mode. 
If TLSCertFile and TLSKeyFile are not used in config but TLSOnly is enabled, the HTTPS dashboard does not loads.

Tested on Debian Linux and Windows Server 2012.
cmd/frps/root.go
// Copyright 2018 fatedier, fatedier@gmail.com
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package main

import (
	"fmt"
	"os"

	"github.com/fatedier/frp/pkg/auth"
	"github.com/fatedier/frp/pkg/config"
	"github.com/fatedier/frp/pkg/util/log"
	"github.com/fatedier/frp/pkg/util/util"
	"github.com/fatedier/frp/pkg/util/version"
	"github.com/fatedier/frp/server"

	"github.com/spf13/cobra"
)

const (
	CfgFileTypeIni = iota
	CfgFileTypeCmd
)

var (
	cfgFile     string
	showVersion bool

	bindAddr          string
	bindPort          int
	bindUDPPort       int
	kcpBindPort       int
	proxyBindAddr     string
	vhostHTTPPort     int
	vhostHTTPSPort    int
	vhostHTTPTimeout  int64
	dashboardAddr     string
	dashboardPort     int
	dashboardUser     string
	dashboardPwd      string
	enablePrometheus  bool
	assetsDir         string
	logFile           string
	logLevel          string
	logMaxDays        int64
	disableLogColor   bool
	token             string
	subDomainHost     string
	tcpMux            bool
	allowPorts        string
	maxPoolCount      int64
	maxPortsPerClient int64
	tlsOnly           bool
)

func init() {
	rootCmd.PersistentFlags().StringVarP(&cfgFile, "config", "c", "", "config file of frps")
	rootCmd.PersistentFlags().BoolVarP(&showVersion, "version", "v", false, "version of frps")

	rootCmd.PersistentFlags().StringVarP(&bindAddr, "bind_addr", "", "0.0.0.0", "bind address")
	rootCmd.PersistentFlags().IntVarP(&bindPort, "bind_port", "p", 7000, "bind port")
	rootCmd.PersistentFlags().IntVarP(&bindUDPPort, "bind_udp_port", "", 0, "bind udp port")
	rootCmd.PersistentFlags().IntVarP(&kcpBindPort, "kcp_bind_port", "", 0, "kcp bind udp port")
	rootCmd.PersistentFlags().StringVarP(&proxyBindAddr, "proxy_bind_addr", "", "0.0.0.0", "proxy bind address")
	rootCmd.PersistentFlags().IntVarP(&vhostHTTPPort, "vhost_http_port", "", 0, "vhost http port")
	rootCmd.PersistentFlags().IntVarP(&vhostHTTPSPort, "vhost_https_port", "", 0, "vhost https port")
	rootCmd.PersistentFlags().Int64VarP(&vhostHTTPTimeout, "vhost_http_timeout", "", 60, "vhost http response header timeout")
	rootCmd.PersistentFlags().StringVarP(&dashboardAddr, "dashboard_addr", "", "0.0.0.0", "dasboard address")
	rootCmd.PersistentFlags().IntVarP(&dashboardPort, "dashboard_port", "", 0, "dashboard port")
	rootCmd.PersistentFlags().StringVarP(&dashboardUser, "dashboard_user", "", "admin", "dashboard user")
	rootCmd.PersistentFlags().StringVarP(&dashboardPwd, "dashboard_pwd", "", "admin", "dashboard password")
	rootCmd.PersistentFlags().BoolVarP(&enablePrometheus, "enable_prometheus", "", false, "enable prometheus dashboard")
	rootCmd.PersistentFlags().StringVarP(&logFile, "log_file", "", "console", "log file")
	rootCmd.PersistentFlags().StringVarP(&logLevel, "log_level", "", "info", "log level")
	rootCmd.PersistentFlags().Int64VarP(&logMaxDays, "log_max_days", "", 3, "log max days")
	rootCmd.PersistentFlags().BoolVarP(&disableLogColor, "disable_log_color", "", false, "disable log color in console")

	rootCmd.PersistentFlags().StringVarP(&token, "token", "t", "", "auth token")
	rootCmd.PersistentFlags().StringVarP(&subDomainHost, "subdomain_host", "", "", "subdomain host")
	rootCmd.PersistentFlags().StringVarP(&allowPorts, "allow_ports", "", "", "allow ports")
	rootCmd.PersistentFlags().Int64VarP(&maxPortsPerClient, "max_ports_per_client", "", 0, "max ports per client")
	rootCmd.PersistentFlags().BoolVarP(&tlsOnly, "tls_only", "", false, "frps tls only")
}

var rootCmd = &cobra.Command{
	Use:   "frps",
	Short: "frps is the server of frp (https://github.com/fatedier/frp)",
	RunE: func(cmd *cobra.Command, args []string) error {
		if showVersion {
			fmt.Println(version.Full())
			return nil
		}

		var cfg config.ServerCommonConf
		var err error
		if cfgFile != "" {
			var content []byte
			content, err = config.GetRenderedConfFromFile(cfgFile)
			if err != nil {
				return err
			}
			cfg, err = parseServerCommonCfg(CfgFileTypeIni, content)
		} else {
			cfg, err = parseServerCommonCfg(CfgFileTypeCmd, nil)
		}
		if err != nil {
			return err
		}

		err = runServer(cfg)
		if err != nil {
			fmt.Println(err)
			os.Exit(1)
		}
		return nil
	},
}

func Execute() {
	if err := rootCmd.Execute(); err != nil {
		os.Exit(1)
	}
}

func parseServerCommonCfg(fileType int, source []byte) (cfg config.ServerCommonConf, err error) {
	if fileType == CfgFileTypeIni {
		cfg, err = config.UnmarshalServerConfFromIni(source)
	} else if fileType == CfgFileTypeCmd {
		cfg, err = parseServerCommonCfgFromCmd()
	}
	if err != nil {
		return
	}
	cfg.Complete()
	err = cfg.Validate()
	if err != nil {
		err = fmt.Errorf("Parse config error: %v", err)
		return
	}
	return
}

func parseServerCommonCfgFromCmd() (cfg config.ServerCommonConf, err error) {
	cfg = config.GetDefaultServerConf()

	cfg.BindAddr = bindAddr
	cfg.BindPort = bindPort
	cfg.BindUDPPort = bindUDPPort
	cfg.KCPBindPort = kcpBindPort
	cfg.ProxyBindAddr = proxyBindAddr
	cfg.VhostHTTPPort = vhostHTTPPort
	cfg.VhostHTTPSPort = vhostHTTPSPort
	cfg.VhostHTTPTimeout = vhostHTTPTimeout
	cfg.DashboardAddr = dashboardAddr
	cfg.DashboardPort = dashboardPort
	cfg.DashboardUser = dashboardUser
	cfg.DashboardPwd = dashboardPwd
	cfg.EnablePrometheus = enablePrometheus
	cfg.LogFile = logFile
	cfg.LogLevel = logLevel
	cfg.LogMaxDays = logMaxDays
	cfg.SubDomainHost = subDomainHost
	cfg.TLSOnly = tlsOnly

	// Only token authentication is supported in cmd mode
	cfg.ServerConfig = auth.GetDefaultServerConf()
	cfg.Token = token
	if len(allowPorts) > 0 {
		// e.g. 1000-2000,2001,2002,3000-4000
		ports, errRet := util.ParseRangeNumbers(allowPorts)
		if errRet != nil {
			err = fmt.Errorf("Parse conf error: allow_ports: %v", errRet)
			return
		}

		for _, port := range ports {
			cfg.AllowPorts[int(port)] = struct{}{}
		}
	}
	cfg.MaxPortsPerClient = maxPortsPerClient
	cfg.DisableLogColor = disableLogColor
	return
}

func runServer(cfg config.ServerCommonConf) (err error) {
	log.InitLog(cfg.LogWay, cfg.LogFile, cfg.LogLevel, cfg.LogMaxDays, cfg.DisableLogColor)

	if cfgFile != "" {
		log.Info("frps uses config file: %s", cfgFile)
	} else {
		log.Info("frps uses command line arguments for config")
	}

	svr, err := server.NewService(cfg)
	if err != nil {
		return err
	}
	log.Info("frps started successfully")
	svr.Run()
	return
}

// Copyright 2018 fatedier, fatedier@gmail.com
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package main

import (
	"fmt"
	"os"

	"github.com/fatedier/frp/pkg/auth"
	"github.com/fatedier/frp/pkg/config"
	"github.com/fatedier/frp/pkg/util/log"
	"github.com/fatedier/frp/pkg/util/util"
	"github.com/fatedier/frp/pkg/util/version"
	"github.com/fatedier/frp/server"

	"github.com/spf13/cobra"
)

const (
	CfgFileTypeIni = iota
	CfgFileTypeCmd
)

var (
	cfgFile     string
	showVersion bool

	bindAddr             string
	bindPort             int
	bindUDPPort          int
	kcpBindPort          int
	proxyBindAddr        string
	vhostHTTPPort        int
	vhostHTTPSPort       int
	vhostHTTPTimeout     int64
	dashboardAddr        string
	dashboardPort        int
	dashboardUser        string
	dashboardPwd         string
	enablePrometheus     bool
	assetsDir            string
	logFile              string
	logLevel             string
	logMaxDays           int64
	disableLogColor      bool
	token                string
	subDomainHost        string
	tcpMux               bool
	allowPorts           string
	maxPoolCount         int64
	maxPortsPerClient    int64
	tlsOnly              bool
	dashboardTLSMode     bool
	dashboardTLSCertFile string
	dashboardTLSKeyFile  string
)

func init() {
	rootCmd.PersistentFlags().StringVarP(&cfgFile, "config", "c", "", "config file of frps")
	rootCmd.PersistentFlags().BoolVarP(&showVersion, "version", "v", false, "version of frps")

	rootCmd.PersistentFlags().StringVarP(&bindAddr, "bind_addr", "", "0.0.0.0", "bind address")
	rootCmd.PersistentFlags().IntVarP(&bindPort, "bind_port", "p", 7000, "bind port")
	rootCmd.PersistentFlags().IntVarP(&bindUDPPort, "bind_udp_port", "", 0, "bind udp port")
	rootCmd.PersistentFlags().IntVarP(&kcpBindPort, "kcp_bind_port", "", 0, "kcp bind udp port")
	rootCmd.PersistentFlags().StringVarP(&proxyBindAddr, "proxy_bind_addr", "", "0.0.0.0", "proxy bind address")
	rootCmd.PersistentFlags().IntVarP(&vhostHTTPPort, "vhost_http_port", "", 0, "vhost http port")
	rootCmd.PersistentFlags().IntVarP(&vhostHTTPSPort, "vhost_https_port", "", 0, "vhost https port")
	rootCmd.PersistentFlags().Int64VarP(&vhostHTTPTimeout, "vhost_http_timeout", "", 60, "vhost http response header timeout")
	rootCmd.PersistentFlags().StringVarP(&dashboardAddr, "dashboard_addr", "", "0.0.0.0", "dasboard address")
	rootCmd.PersistentFlags().IntVarP(&dashboardPort, "dashboard_port", "", 0, "dashboard port")
	rootCmd.PersistentFlags().StringVarP(&dashboardUser, "dashboard_user", "", "admin", "dashboard user")
	rootCmd.PersistentFlags().StringVarP(&dashboardPwd, "dashboard_pwd", "", "admin", "dashboard password")
	rootCmd.PersistentFlags().BoolVarP(&enablePrometheus, "enable_prometheus", "", false, "enable prometheus dashboard")
	rootCmd.PersistentFlags().StringVarP(&logFile, "log_file", "", "console", "log file")
	rootCmd.PersistentFlags().StringVarP(&logLevel, "log_level", "", "info", "log level")
	rootCmd.PersistentFlags().Int64VarP(&logMaxDays, "log_max_days", "", 3, "log max days")
	rootCmd.PersistentFlags().BoolVarP(&disableLogColor, "disable_log_color", "", false, "disable log color in console")

	rootCmd.PersistentFlags().StringVarP(&token, "token", "t", "", "auth token")
	rootCmd.PersistentFlags().StringVarP(&subDomainHost, "subdomain_host", "", "", "subdomain host")
	rootCmd.PersistentFlags().StringVarP(&allowPorts, "allow_ports", "", "", "allow ports")
	rootCmd.PersistentFlags().Int64VarP(&maxPortsPerClient, "max_ports_per_client", "", 0, "max ports per client")
	rootCmd.PersistentFlags().BoolVarP(&tlsOnly, "tls_only", "", false, "frps tls only")
	rootCmd.PersistentFlags().BoolVarP(&dashboardTLSMode, "dashboard_tls_mode", "", false, "dashboard tls mode")
	rootCmd.PersistentFlags().StringVarP(&dashboardTLSCertFile, "dashboard_tls_cert_file", "", "", "dashboard tls cert file")
	rootCmd.PersistentFlags().StringVarP(&dashboardTLSKeyFile, "dashboard_tls_key_file", "", "", "dashboard tls key file")
}

var rootCmd = &cobra.Command{
	Use:   "frps",
	Short: "frps is the server of frp (https://github.com/fatedier/frp)",
	RunE: func(cmd *cobra.Command, args []string) error {
		if showVersion {
			fmt.Println(version.Full())
			return nil
		}

		var cfg config.ServerCommonConf
		var err error
		if cfgFile != "" {
			var content []byte
			content, err = config.GetRenderedConfFromFile(cfgFile)
			if err != nil {
				return err
			}
			cfg, err = parseServerCommonCfg(CfgFileTypeIni, content)
		} else {
			cfg, err = parseServerCommonCfg(CfgFileTypeCmd, nil)
		}
		if err != nil {
			return err
		}

		err = runServer(cfg)
		if err != nil {
			fmt.Println(err)
			os.Exit(1)
		}
		return nil
	},
}

func Execute() {
	if err := rootCmd.Execute(); err != nil {
		os.Exit(1)
	}
}

func parseServerCommonCfg(fileType int, source []byte) (cfg config.ServerCommonConf, err error) {
	if fileType == CfgFileTypeIni {
		cfg, err = config.UnmarshalServerConfFromIni(source)
	} else if fileType == CfgFileTypeCmd {
		cfg, err = parseServerCommonCfgFromCmd()
	}
	if err != nil {
		return
	}
	cfg.Complete()
	err = cfg.Validate()
	if err != nil {
		err = fmt.Errorf("Parse config error: %v", err)
		return
	}
	return
}

func parseServerCommonCfgFromCmd() (cfg config.ServerCommonConf, err error) {
	cfg = config.GetDefaultServerConf()

	cfg.BindAddr = bindAddr
	cfg.BindPort = bindPort
	cfg.BindUDPPort = bindUDPPort
	cfg.KCPBindPort = kcpBindPort
	cfg.ProxyBindAddr = proxyBindAddr
	cfg.VhostHTTPPort = vhostHTTPPort
	cfg.VhostHTTPSPort = vhostHTTPSPort
	cfg.VhostHTTPTimeout = vhostHTTPTimeout
	cfg.DashboardAddr = dashboardAddr
	cfg.DashboardPort = dashboardPort
	cfg.DashboardUser = dashboardUser
	cfg.DashboardPwd = dashboardPwd
	cfg.EnablePrometheus = enablePrometheus
	cfg.DashboardTLSCertFile = dashboardTLSCertFile
	cfg.DashboardTLSKeyFile = dashboardTLSKeyFile
	cfg.DashboardTLSMode = dashboardTLSMode
	cfg.LogFile = logFile
	cfg.LogLevel = logLevel
	cfg.LogMaxDays = logMaxDays
	cfg.SubDomainHost = subDomainHost
	cfg.TLSOnly = tlsOnly

	// Only token authentication is supported in cmd mode
	cfg.ServerConfig = auth.GetDefaultServerConf()
	cfg.Token = token
	if len(allowPorts) > 0 {
		// e.g. 1000-2000,2001,2002,3000-4000
		ports, errRet := util.ParseRangeNumbers(allowPorts)
		if errRet != nil {
			err = fmt.Errorf("Parse conf error: allow_ports: %v", errRet)
			return
		}

		for _, port := range ports {
			cfg.AllowPorts[int(port)] = struct{}{}
		}
	}
	cfg.MaxPortsPerClient = maxPortsPerClient
	cfg.DisableLogColor = disableLogColor
	return
}

func runServer(cfg config.ServerCommonConf) (err error) {
	log.InitLog(cfg.LogWay, cfg.LogFile, cfg.LogLevel, cfg.LogMaxDays, cfg.DisableLogColor)

	if cfgFile != "" {
		log.Info("frps uses config file: %s", cfgFile)
	} else {
		log.Info("frps uses command line arguments for config")
	}

	svr, err := server.NewService(cfg)
	if err != nil {
		return err
	}
	log.Info("frps started successfully")
	svr.Run()
	return
}

Yes. https://go.dev/blog/gofmt