How to use a workflow for Special Order PO Created from Work Order has a specific document number?

We would like the special order PO created from a work order has the same document number as the work order. I think a workflow should be able to achieve it.

Create a new set field value action. I would like the PO {tranid} = {createdfrom.tranid}

But in Set Field Value field drop down list, I can't find which field is tranid. There is no Document Number. I tested to choose Order #, Transaction, Transaction Number. But none of them work.

Is it possible to use a workflow to set up special order PO document number the same as work order?