It's using either the name of your drive/volume or the name of your computer as your default username, which to me is very silly, and what's worse is that it doesn't "remember" the values you enter on that screen. It would seem like it should use your profile or "remember" - but it doesn't seem to do either.
You might try the 'Play Now' option unless you're truly going for a custom game, since unless you're truly customizing the end results are the same.