How can I ask onboarding questions as part of a payment form?

There are two ways.

The recommended way is to keep the payment on a two step order form and, after the form is submitted, redirect the buyer to a second page with an onboarding form. Set the redirect URL (you can do this in the funnel builder). On the onboarding form ask for what you need up front, such as their website address, logos or other items your service needs.

Alternatively, put the Payment element in a survey form. A survey is made of screens of questions (screen one, screen two and so on), so you can ask questions before or after the payment on one form.