If this is an X15 R1 (the XPS is a totally different Dell system), it won't have a legacy boot mode -- it's UEFI only. And if it's a new XPS 15 (9520,etc.) it will also be UEFI only. That means that whatever OS you want to load on the external drive must be fully UEFI compliant. Any of the USB ports will work with an external boot device.  Before you do any more changing of setup options, with the drive you want to use connected, press F12 at powerup.  If the system sees the drive as bootable, it will appear in the boot menu that shows up.  If you do not see it, it is not UEFI-bootable. There is no real need to set the drive in your setup list -- just press F12 when you want to boot from the external drive.  If you're trying to disable secure boot,  use the disable all keys setting.  
I just bught an Alienware XPS15 R1, and am trying to set up a bootable external drive.  I'm new at this, so please excuse the basic questions.  I have tried researching for answers on various websites, and can't find a step-by-step procedure to make the correct changes to the BIOS. System Info shows Bios as BIOS Version/Date Alienware 1.10.2, 5/17/2022, SMBIOS Ver 3.3, but UEFI shows Aptio Setup Utility, Bios Version 1.10.2 1. I have a USB 3.2 gen 1 port and a USB-C 3.2 Gen 2 port.  My HD uses the 1st port.  Can I use that for my bootable device, or do I need a connector for the 2nd port? 2.  Some of the "solutions" I read say that BitLocker must be  turned off, Secure Boot must be off, and Legacy Boot must be turned on.  Other solutions indicate just the opposite.  Under my Security tab in the Bios, Secure Boot has no "Disable"; just several options: PK, KEK, db, dbx, Reset all Keys, Delete all Keys.  How do I disable? 3. Under the Boot tab, I have only Boot Option #1 [Windows Boot Manager], or it can be disabled.  Which should it be? If anyone can point me to a link that has detailed (and correct) information, I would be very thankful!  
How to change Bios for bootable external HD