I have two UserApp instances in the same eDir

one for external users to access via an external(www)-facing web portal
one for internal users to access via portal on company intranet

I'm wondering how to start an internal UserApp workflow from an external UserApp PRD.

I've started other workflows from workflows within the same UserApp, so I understand the general idea, but this little twist on the theme isnt't working.

I'm entering the full DN of the PRD I want to start into the "Provisioning Request Defn to start" field, but not sure what to do from there. I have tried manually adding the Source and Target info, but any attempt to start the internal UserApp's PRD from the external UserApp just returns errors like "Workflow Error: Dataitem [<any applicable target form field I try>] was not found in request."

Does anyone have any ideas on what I can try/do differently? Any assistance is greatly appreciated.