Select the tournament you wish to register with.
You will see the tournament information collumn on the right side of your screen.
If you see a register button in that space, it means you still have time to register for this tournament.
You can also double-click on the tournament name and the tournament lobby will open up.
There will be a register button is this lobby too.
Click on the register button for the designated tourney.
Once you are registered, the register button become the unregister button.
You are able to unregister until the tournament starts his seating mode.
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