Affiliate Marketing Secrets

This question probably has the easiest answer ever, but on Twitch, i’ve been seeing these affiliate banners at the bottom of user’s channels, asking you to ‘…buy games from this banner to support my channel…” So I signed up for the affiliate (which gives you [i think] 10% off). But when I click on other user’s banners, the link at the top says ‘…partner-[THEIR USERNAME]…’ So I was wondering how to change it, because right now mine is just ‘…partner-user[a bunch of numbers]…’
If you can tell me how that’d be great!! Thanks! :]