Why do you need a login system, if to play on demo servers, users must create an account here on LFS?
There are only two possibilities:
- You are trying to connect users account to an external system, like website, mobile app, or whatever OR
- You are hosting a cracked server, based on IP-Patcher, where the user can put anything he want on Username without a Master Server confirmation.
My bet is the second option. So, if you are hosting a cracked server, at least don't come to official game forum to request help for it.