Losing my mind here-- on a SO Line - want a drop down to select another SO document number — i.e a valid currently existing SO to point to for reference.

Can't you just use the

Sourcing and Filtering

options on the definition of the dropdown?

unfortunately not. I have to do it as free-form text in order to source from Source:Sales Order and source from:Transaction Number --> So it does not give the user a dropdown to validate the sales order number