I have a requirement for the SO302000 (Shipment) screen. In Classic UI, I added a read-only display field at the top of the SOShipLineSplit (Line Details) popup to show the total quantity of all split lines.

This field does NOT need to be stored in the database — it's just a simple display label for user reference.
I have issues when migrating it from classic UI, but there is no need to migrate, if I can develop it directly in Modern UI.
How can I implement this in Modern UI? I just need to show a calculated total qty field on the popup header.

