Tax code and Tax jurisdiction codes in ML81N

[vancouverannie]

We create service purchase orders with unplanned services and indicate the tax code and tax jurisdiction code on the purchase order.  When doing a service entry (ML81N) the tax codes/jurisdiction codes do not always populate the tax code/jurisdiction codes that are indicated in the purchase order.  Especially if there are multiple lines on the PO with differing tax codes/jurisdiction codes.  Is there a BADI or 'something' that would force the tax code/jurisdiction codes from the referenced PO/line item into the service entry sheet?  My Programmer can't seem to make it work...any assistance would be appreciated.!!