How do i add the clan name to my username and in-game name pls

You can't add your clan tag to your username.
It has to be kept like that.
But to change it in game, either go to Login and type [yourclan]RadollDie to the nickname and password and login.
If you want to make it via command, then press t and type /lo [yourclan]RadollDie password

And these questions goes to Support forum, not in FAQ.