func (s *PullRequestsService) RequestReviewers(ctx context.Context, owner, repo string, number int, reviewers ReviewersRequest) (*PullRequest, *Response, error) {
	u := fmt.Sprintf("repos/%s/%s/pulls/%d/requested_reviewers", owner, repo, number)
	req, err := s.client.NewRequest("POST", u, &reviewers)
	if err != nil {
		return nil, nil, err
	}
	r := new(PullRequest)
	resp, err := s.client.Do(ctx, req, r)
	if err != nil {
		return nil, resp, err
	}
	return r, resp, nil
}
RequestReviewers creates a review request for the provided reviewers for the specified pull request.

GitHub API docs: https://developer.github.com/v3/pulls/review_requests/#create-a-review-request
package github
import (
	"context"
	"fmt"
)
type ReviewersRequest struct {
	Reviewers     []string `json:"reviewers,omitempty"`
	TeamReviewers []string `json:"team_reviewers,omitempty"`
}
type Reviewers struct {
	Users []*User `json:"users,omitempty"`
	Teams []*Team `json:"teams,omitempty"`
}