Auto-populate an item list that is populated from a custom field in another doctype


I have a Custom Field in the Delivery Note Items doctype. This is reference in the Packaging Slip Items list. It works correctly, but will only populate the data from the Custom Field after you click the down arrow button on each line item.

Is there a way when the Packing Slip form is opened, it automatically fetches the data from the custom field in the delivery Note Items and populates in the Packaging Slip Items table without having to click each individual item?

Upon opening the packing slip form.

Only will populate after you click the down arrow (pulls from custom field in delivery note):

Is there a place where I could put a script in to auto populate once the form is opened?