Coming soon. Choosing a Kit form per product will be part of the Zapier-based Kit connection. This page will cover it once that's live.
What this will do
You'll be able to control which Kit (ConvertKit) form each buyer is added to after they pay — so different products can feed different lists, tags, or automated sequences.
In the meantime
You can already control this inside your Zap. In Kit's Add Subscriber action you choose the form (or tag) the buyer is added to, so you can build one Zap per product if you want different forms. See Connect Kit via Zapier and Connect Zapier to bookto checkout.