Adding AP Line extension fields to GL postings

I am trying to get an extension field that I have added to an AP Invoice Line to show on the resultant line in the GL. As a standard extension field there is no option to do this but I have seen on an earlier version that someone had added a script to a field that allowed it to be reflected on the resultant GL line (in the Self-Enablement system, 319968, when it was version 1411).

Please could someone share how this can be achieved.

Many thanks

Alastair