To find a player's handles (or ID) use https://sc2arcade.com/profile-list and search for their name, this allows you to view their profile and copy their ID, for say reporting purposes.

It's also useful for looking up a player's punishments using: https://www.sc2mafia.com/forum/banmanager.php?uid=

This method is an easier alternative to looking through your game files to find your own handles, which can be a bit tedious.

My profile for example (omg reported):
image_2022-04-04_185728.png