First set few action in case its coin set the amount of coins and set as action and add a name.Now create a separate UI for store and add a purchase button.Select in-app from right bottom add bundle id,tick buy and use,and aboe select the following action that you set before in drop down.And add an image to the purchase button.
Hey,follow the above method and instead of coins add powerup option from drop down and purchase action as an IAP.