You will need a temporary username and password to play. If someone already created a match, select it in the "create new user" form.
If you want to create a new match yourself, select the option "Create new..." and enter a name for it.
If you were logged out of a match, you can re-enter via the login form.
Username and passwords are only stored for the duration of the game,
there is no need to use a strong password.