To compete in a Multiplayer Champions Cup you need to register for that cup in advance.
Each cup has a registration period before the cup commences.
You will receive a notification and your assistant will let you know when registration is open.
During the registration period, you can also cancel your registration from the cup's registration screen (Squad Management screen) 'Menu' button. Tap on the cup menu panel to access the Squad Management screen.
If you try to register for a second multiplayer cup, you will be given the option to cancel your registration for the first competition.
You can only participate in one multiplayer cup at a time.