Using Username and Password with uNET

Okay, so this is something I'll have to tackle soon, and I'm not really sure where to start.I'm not very familiar with servers yet, but I'm getting there. Thing I can't seem to figure out is this.

Say I have a matchmaking server and I'm using Unity 5.x to connect to that server for my game. How could I set up the server to hold usernames and passwords so that they are accessible from JavaScript within Unity?

I'd like to have people log in to my game and have variables tied to their specific user account activate upon connection, like say there position for example...

I have a scene set up with GUI input fields for username and password, but that's about as far as I've gotten really... This new network setup has honestly been confusing me a bit. Anyone have any suggestions? At least something to set me on the right path?