brotherton-erpnext/erpnext/docs/user/manual/en/customize-erpnext/custom-scripts/custom-script-examples/custom-script-fetch-values-from-master.md

To pull a value of a link on selection, use the add_fetch method.

add_fetch(link_fieldname, source_fieldname, target_fieldname)

Example

You create Custom Field VAT ID (vat_id) in Customer and Sales Invoice and want to make sure this value gets updated every time you select a Customer in a Sales Invoice.

Then in the Sales Invoice Custom Script, add this line:

cur_frm.add_fetch('customer','vat_id','vat_id')

{next}