The Anti-Ship Challenge is a challenge in which you write a fic or draw fanart for a ship that you do not support, and if possible, the one you dislike the most (i.e. R/Hr shippers would probably do H/Hr fics/art for this challenge).
What if I decide to cheat and do my favorite ship instead?
You do that. We won't know. But it's more fun if you honestly choose your "anti-ship" and go out from there.