Machine Center Task List Sort....

Hi all… I am trying to add an auto sort on open for the Machine Center Task List form. This form pulls info from the Prod Order Routing Line table. I have also linked a column on the form to the Production Order table in order to get “Due Date” for the items to show. My question is how do I sort on that “Due Date” field when it is not an actual field from the main table that the form is pulling from? I tried SETCURRENTKEY(ProdOrder.“Due Date”); but that did not work at all. There is no “Due Date” in the Prod Order Routing Line table so I cannot set is as a key i guess? Should I just a another field to that table and have it pull the date over from the Production Order table and then set it as a key and sort? Hope that made sense…Any help? Thanks, JP