Hey Steven. Typically, I'll add the path to the PATH Environment variable so that I can just type "tabadmin" without the full path each time. It makes life easier.
But, in your case using Powershell, it won't kick off your command until you do what it said in the text at the bottom, and either reference the exe file via full path, or relative path:
.\tabadmin set wgserver.trusted_hosts 10.0.0.13
(note also that it's not "wg.server" but "wgserver"--I've edited my reply to account for that and another typo)