Why couldn't you use a Column-Post Validate event on the ItemCode field to fill the PO No. field for your conditions. It would only fire once per ItemCode and not when otherwise changing the line. 

Column Post Validate on ItemCode should work if on a version that allows selecting it (older versions didn't but I think 2020+ does in a detail script).

A table Post Read event filtered by oBusObj.EditState = 2 also works and once the record is saved, the EditState will be equal to 1.

Another event could be the Set Default Values one, I haven't used it much but it is supposed to run when default values are being set by the business object on new records.

Column Post-Validate is in every version since script events were introduced in 2010 with version 4.40. I think it is your safest bet because any event that precedes setting the ItemCode will cause the PO number field to be cleared.

Column Post-Validate is in every version since script events were introduced in 2010 with version 4.40. I think it is your safest bet because any event that precedes setting the ItemCode will cause the PO number field to be cleared.

The event existed since 4.4 but in certain tables, certain columns were not selectable on column events.  I seem to recall at one point, the ItemCode column could not be selected on detail tables.  I can't recall what version was the last version I saw this on though, I did just check my 2018 install and it can be selected so it may have been corrected in 2018 or maybe 2017.