@IAINADAMSON

You can’t render another application’s harness inside a modal that runs under the current access group, so don’t use Harness-in-a-modal for this. Instead, on the button click use Open URL in Window (new window) and deep-link into App B. In App B, create a lightweight landing harness (or portal) whose pre-activity switches to App B’s access group if needed (pxSwitchApplication) and then loads pyReviewArchivedCase with the passed case ID. Build the URL to that harness/portal and include the archived case key as a parameter. Make sure the operator has an access group for App B and that authentication is shared (same SSO session) so the deep link opens without a new login. In the pre-activity, validate access and call the OOTB open-archived-case API you use today, then Show-Harness pyReviewArchivedCase. Configure the button to open as a pop-up/new window and set window features as needed. This approach keeps your current portal intact while the pop-up runs in App B’s context, with correct rules/app stack and security.