Yes you can, but it is discouraged for your own benefit. Send an email to the assignor at firstname.lastname@example.org . However, if you choose to work only as an Assistant Referee, the assignor will have to changed your ranking in Arbiter Sports and it will move you to the very bottom of the assigning list. Since all games need centers and only about 20% – 30% of our games use ARs, you may not get any assignments. As a result, you won’t get the opportunity to mature as a referee or assistant referee. Please think about your decision before emailing the assignor.