UDF in Purchase Order Screen with Selector Box for ARInvoice.RefNbr

I'm trying to create a UDF in the Purchase Order screen (PO301000) that is a selector box populated with ARInvoice.RefNbr.  The business case is to be able to tie particular purchase orders to an AR Invoice where we submitted for warranty reimbursement so we can track outstanding warranty orders.  I created my attribute with the selector box with schema field PX.Objects.AR.ARInvoice.RefNbr but it doesn't populate within the PO301000 screen.  The reason is that RefNbr is tied to the ARInvoice.DocType field and there's not a way to select that within the purchase order screen.  I'm wondering if there's a customization where I can hard code the selector box to ARInovice.DocType = 'INV' so the AR Invoices will populate in the UDF?

First pic is the PO screen with the UDF as I tried to make it, nothing is populating in the selector box.  Second pic is the Attribute I created for the UDF.  Appreciate all any help

Thanks,

Eric