Hi,
I think you want to open the manual task at the top branch of the AND split?
Clicking on open form will open a page like this:
And then, go back to the process report, open Parameters tab, go to Internal parameters, and look for "P_document", and it should have "id" node.
This id is the string after "?instance" in the URL above.
In your case, since you have an AND split, there should be 2 sets of P_documents, each under a parent node with branch id (the activity id of the immediate next activity after the AND split).
One of it will have the manual task, and that branch's P_document's id will give you the URL you want.
Can you try this?
Best regards,
Taka