At this time, Memberstack doesn't allow for direct custom field additions to the Stripe checkout page. However, you can incorporate custom fields into the signup form before a member opts for a plan. Additionally, you can use a profile form to update custom fields after purchase. For guidance on setting up a profile form in Webflow, check out this article.
Should you have any more inquiries or need further assistance, feel free to reach out by submitting a Support Request.