You have a lot going against you in this game. The ball is over-inflated, the hoop is smaller than regulation size and often an oval shape rather than circular. The backboard is plywood making it extra bouncy. So forget about throwing a normal free-throw shot.
The key is to use a high arc. Do not try to rebound the shot off the backboard — the ball will always bounce too much. To win this game you must make a perfect swish, no backboard.