[2023] How do you make a serveradmin.xml file?

Originally posted by Soubi: And if I wanted to add someone with a custom URL, like mine (SoubixLoveless) do I just put that instead of the numbers. Or would the ID64 be better?

You need to get their SteamID in ID64 format, the name is not correct for the config file.
The easiest way is to indirectly exercise the public Steam API by using 3rd party sites to do the lookup for you. (Enhanced Steam Browser Addon also displays the numeric SteamID64)

Example:
http://steamcommunity.com/id/RedEagleLXIX/
Go To: http://steamrep.com/
paste the URL http://steamcommunity.com/id/RedEagleLXIX/ and search
result: steamID64: http://steamcommunity.com/profiles/76561197971689303
So to make me an admin one would use: 76561197971689303