Triggering a Zapier Task from a Store Transaction

I understand that I am able to add Google integration and Zapier tasks to forms that I create on my content pages.  However, I have created custom forms on my products that I sell through the store.  I do not see the familiar "Storage" tab on the custom form page to connect the submission of the form to an external integration.  Does anyone know how I can accomplish this in Squarespace?  I want to use the successful purchase of a product as a trigger to execute a Zapier task.