[usrusr]

I once tried doing the prudent thing and give them an individualized email address on a catchall subdomain, now my user name on Windows 11 is "win10". Because why ask for a username if you have an email address, right? Might get interesting when your email is administrator@ or guest@, I don't get the impression that anyone at Microsoft has even the slightest idea what actually goes on in their schizophrenic SSO multiverse.

[Uvix]

It only uses the first 5 letters so administrator should be fine (“admin” isn’t a built-in). Guest could be interesting, though.