Restrict username to user first name

[Lilly]

you posted this already I think

How to display user first name as username

I prefer the username to consist of the user’s first name rather than their full name. Upon the user’s initial login, I would like the system to automatically set the username using only the first name. E.G Full name: Maria Jose → username: Maria So instead of the username be Maria_Jose, and want it to be Maria How I can accomplish that?