Muay Thai Championship Series 2020
Go to Setup - Login, and just type the clan name in before your name, and put brackets round it like so:

[Clan]Username

And then log in.
[19:39] <Birdflu> I'm just sad that I can't give myself one
[19:39] <Birdflu> I'd have a great time
First you have to get accepted into the clan.

Then, you can go to the login, and add the [CLAN]before your name.

Or ingame type /login [CLAN]Username password
relieve my heart of malice