Try this script set to perform via OnObjectExit on the description field:
Set [LineItems:: PartNumber ; PriceListByDescription:: PartNumber ]
I haven't tested it and you may have a problem with the portal row not yet being committed, but if it works, it should fill in the part number for you automatically.
You have saved me once again.
It works like a charm, it causes the description to be looked up as well so if the description is entered in all lower case when you exit the field the description changes to the correct format of upper and lower case that is in the price list.
(I removed the auto enter lookup for the Part number)