In the stripe.com dashboard:
And now, in Mendix you do not need to validate if the payment was successful. Rather, landing on this page means that the payment was successful. You can also include the confirmation-session-id as parameter to that link. See for more info on this: https://stripe.com/docs/payments/payment-links#change-confirmation-behavior. Have not yet gotten around myself to working that out, but it looks like it has everything you might need.