Pinpointing proof comments via API

Anybody advise on how to/ if it’s possible to pinpoint proof comments when creating them in Fusion? The rest API lets me narrow them down to page X but I need to go a stage further and pinpoint them to specific X and Y co-ordinates with the arrow as if the user was doing it via the UI.

I’ve tried recreating the calls the UI makes but I can’t make it work. It calls https://us.my.workfront.com/proof/rpc/index.php and passes a session ID in the request body but both the session ID returned via the ProofHQ rest API and the main WF API result in errors when creating comments.