To pull a value of a link on selection, use the add_fetch method.
add_fetch(link_fieldname, source_fieldname, target_fieldname)
Example
Let' say you have created a Custom Field VAT ID (vat_id ) in Customer and Sales Invoice and want to make sure that this value gets updated every time you select a Customer in a Sales Invoice.
To configure this, in the Sales Invoice Custom Script, you can add this line:
cur_frm.add_fetch('customer','vat_id','vat_id')
