Web form that can populate a Product instead of a field

Web form that can populate a Product instead of a field

I run a SaaS business and we have tied Zoho CRM to the subscription sign up process for our service. The process is straight forward. When a new subscriber signs up I want a new lead record created in Zoho that shows what subscription plan ("product") they signed up for.

The problem is that I only see custom or existing data fields that I can add to a web form. I can't add a product from the product module to a form.How do I do this? Does this need to be done with the developer API?