If you don't know already, Radium is a Rec Room custom server, and to access it you need a referral code.
Only some people have access to these referral codes, so it's hard to get them.
If you want a referral code please click the "View list" button, and if you want to add one click the "Add Referral Code" button.
Some of them might not work, keep trying until you find one that works.