Create a sprite "BalloonString" - a horizontal line.
Set origin image point at its left end.
Set position of this sprite to the player's hand and pin it to the player.
Then do this:
On every tick
BalloonString -> set Width to Distance(player.x, player.y, balloon.x, balloon.y)
BalloonString -> set Angle towards position (Balloon.x, balloon.y)