In order to Forfeit, you must be losing. Then you can Forfeit as soon as you gain possession of the ball.
The best way to do it is switch your goalie with your MF. If you had two players on the bench that you never used, then put them in defense, so that they won't be able to steal the ball away from your opponent.
Then have your RF, MF, and RD Mark the opponent's LD, and your LF and LD Mark the opponent's RD. This will open the path for the opponent's MF, LF, and RF to your goal. Easier for them to score. After AI has scored, you can Forfeit.
"We aim to please. You aim too, please." -Janitorial Staff