Patch polymorphic lookup Canvas App to Dynamics CE

I have made a Canvas App linked via CDS to Dynamics 365 CE. Saving a photo from the app to the annotation table in Dynamics is working fine.

Now I would like to link this annotation record to an existing contact, which I have selected in the app using the field objectid in a patch command (I have a GUID, a contact name and an entity name). This objectid is a polymorphic lookup field, but I can't figure out how this objectid should be filled. Any help would be appreciated

Categories:

Connecting to Data