In the first area of the game, after repairing the bridge, you can enter the house on the right of the windmill to find the "baskick champion".
Accept the challenge and when the minigame starts, kick the watermelon three times in your basket on the right to win the game. Takes a bit of practice but very doable.
Best advice is to let the bumper push it away from the wall and kick it where it stops. The distance is often just right.
* 3 wins means 3 entire games won back to back for a total of 9 hoops. Games don't have to be perfect as long as you win with 3-something in the end *