Access to Postmark StrippedTextReply

Is there a way I can extend mailbox to save the StrippedTextReply from postmarks json.
Or am I best calling postmarks api with a matched Message-ID header to then access this?

If I have to do it with the latter is there an easy way to save postmarks MessageID (different from their header Message-ID) in the inbound mail table separately? as the message id passed at the moment is the one that has come from the senders gmail for example.

Thanks