How to Crack Username in Online Form

Hello!

I have a problem. I got WiFi router, but I can't access admin page. My ISP changed default password and probably username, and he don't want to give it to me. I know I can just recover to factory settings, but he told me that I will lose guaranty, so I want to crack it. I want to use Hydra, but it only cracks password and not username. So, is there any way to crack username too?