Stand on the green platform that is before the goal (not on the actual platform that the goal is on, but the one to the left of the goal)with Yoshi and a Cape Feather. Jump off the platform and float under the goal. When you are under the goal, jump off Yoshi to end up on the other side of the goal. Continue to the right to reach the Secret exit.
That''s a funny lookin'' donkey you''re riding! Me wanna ride!
