DO NOT USE (not like you can)
This is an old project of mine that had the goal of beeing the ultimate steam account manager (lost interest lol)
It used to fill out login details (and steam guard codes) by injecting a DLL into steam and doing evrything internally to avoid leaks.
Older methods of doing things leaked usernames and passwords in the steam process arguments and has no good way to pass steam guard codes other than through a simple coppy paste ot simulating keyboard input which has it's own problems.