How to retrieve transcripts sent via messenger via API

We have a marketplace app that uses the Zendesk ZAFClient to subscribe to events and sits within the customer's ticket sidebar. Whenever a new message is sent our app pings off the transcript to our server, and our server processes it. However, it doesn't store the messages once it's done; they are deleted.

Then, our customers have access to a dashboard through our product where they can view conversations from Zendesk. We are using the Chat API and Support API to retrieve these conversations currently. This works fine when Chat and Support are completely separate. But when the customer is using messenger, the messages don't appear in either APIs.

How do I pull the transcript for messenger conversations? Is it through Sunshine somehow? And if so, how do I authenticate with Sunshine using the API token we already have via the Support OAuth flow?

Thanks