I need to add the customers order reference / purchase order number to the delivery slip.

In the "Sales Order" there is a variable called Client_Order_Ref, which comes from Customer Reference box.

This is their own reference for the order. It is printed onto the end invoice, but i need it to be printed on the delivery documentation too.

I am using Base_Report_Designer to edit the reports, I can add custom variables to the Reports and to the Forms, but I there is no variable Client_Order_Ref in the object Stock.Picking.Out. How can I carry this variable through when the Delivery Order is created?