On Huntmania.net you can only log in with a ManiaPlanet account. It's the same account you use for logging in into Trackmania².
Don't worry! We don't save anything of your login information, because - in fact - we aren't able to do that technically. While you log in with your credentials, you are on an official ManiaPlanet page created by Nadeo. We only get the permission to access the most important things for us, like your Trackmania username and your country and region.