Custom Affiliate Id

I am trying to set up my affiliate id to be something other than affiliates=1. I would like it to auto populate it to be affiliates={username} but I can’t seem to figure it out. When I try to change it in the Options tab it says it’s not valid. Is there a way to set this up automatically or can you advise me how to change each affiliate to this as they signup.

Thank you,
Kimberly Payne