Official comment

They are a proprietary website builder, so I'm not sure how they handle integrations.
The best way would be to embed an ONTRAPORT form into the website directly. ONTRAPORT forms use JavaScript. Ask them if that is possible. (I did check their home page and the links at the very bottom to their manual and "how tos" are not working).
I will be moving this message to our regular Question & Answer forum as it does not directly relate to a payment gateway we would incorporate into ONTRAPORT.