how to attach checkout custom form to products through API

I want to create/update products through the API and attach an existing checkout custom form to them. However, I do not know how to do this through API. I can only do this manually. I do not see any documentation on how to attach a checkout form to the product. Looking https://developers.squarespace.com/commerce-apis/create-product
 

Also when I retrieve my products (GET https://api.squarespace.com/1.0/commerce/products), I do not see any info about the checkout custom forms for products that have the form attached to them (which I can see in the UI - see screenshot).