I think you may be getting a little confused about a few things. You can't really use the username to log in. Given that you can change public names every few minutes and can be the exact same as another person. By default, for the gamer card, your User name will be a series of numbers with that most Steam scanning sites will understand. It's part of the URL of your Profile page.. You can change it to something more recognisable URL. Most sites will take custom name over then number.