Accounts
Some mods can use authentication to store persistent game data. For example, Jumpmaze uses this feature to store personal best times and map leaderboards. This requires creating an account on the Zandronum auth server and logging into it from in-game.
Creating an account
Simply go to the register page on the auth website, and create an account. You'll probably have to check your email for a verification link.
Logging in
Menu
You can log in by opening the main menu, then going to Options -> Multiplayer Options -> Log in. Enter your username and password into their respective fields, then select Log in.
Console
To log in via the 
Console, use the login command, like such:
login myusername test123
You can bind it to a key for easy access.
bind kp7 "login myusername test123"
Saving login credentials
On Windows, you can save your login information to the operating system's secure credential storage, so you don't need to type your username password each time. To do so, open the Console, and use the login_add command:
login_add myusername test123
Then, to log in with myusername, you will only need to type:
login myusername
You can also set the default username for the login command, so you can avoid typing your username too:
login_default_user myusername
Then you can just use the login command without any parameters to log in:
login
See also
- Database, for documentation on programming mods with the database and auth functionality