2D catapult game

I want to make a 2D mobile catapult game but I need some help. For firing an object the catapult will have to stretch backwards and then like a spring it will fire the an object. I think I will use the hinge joint with a motor to do the lift of the firing but how can I make it so when I press and hold my finger then the catapult starts to stretch backward?

Thank you in advance.