I have never tried this and will not guarantee it will actually work.
You can launch a window using "Open URL in Window". If you do not use alternative domain, the launched window can display a Pega Harness that references a Section. That Section can access a Data Page that contains an alternate domain URL, plus the information needed for a POST request. The Data Page can source an Activity. The Activity can create an "Interface" step page using the aforementioned information. Specifically that information is used to populate the QueryString or JSON text message body of an HTTP connector's POST request. The response from the POST request would be HTML. The Activity could then call Show-Stream to output that response HTML to the Harness.
Federated Case Management (FCM) enables you to connect PRPC applications. Users in a federation (a group of applications connected by FCM) work in their local application portal, which can display cases and assignments from other applications in the federation. Each local application processes and maintains its own cases and data, and publishes data to a centralized database, the Federated Case Management Repository (FCMR).