A grappling hook is a pretty complex thing to create.
One simple way you might try is to draw a line (or stretch a sprite) from the player to the target. When the line or sprite reaches the target, make it shrink from the other end and set the player's position to the end of it.
Of course, that is a very simple method and it doesn't take into account swinging or collision, but you could expand on it from there.
You may not want to use Platform behavior for your player, especially if you want to swing back and forth. You're looking at some hard math involving sin and cos to do that.