I already tried that

So I was writing an application to join a clan (the army of trolls) and the leader said I can join only if I post a banner. I followed the instructions on youtube and got up to the point where you push okay to paste the banner code where you post a comment. I went up to BBCodes and selected banner, and typed the special code into the options box. I pushed okay and the box disappeared and no code or text was posted. Can someone please help me? thanks