When the tournament has started, it will be possible for you to join the on-site tournament lobby for your first match. This is where the participants of that game will receive the tournament code.
Before that, there’s nothing else you need to do besides making sure that everyone in your team has confirmed their participation, i.e. checked-in.
Learn how to use a Tournament Code here.
Running into other issues, or do you have additional questions? Feel free to chat with us by clicking (?) on Challengermode.