Tick Apply Tax Withholding Amount on unsaved customer receipts whose
references include a Sales Invoice carrying withholding: in the
get_payment_entry override (Create > Payment), in the
allocate_amount_to_references doc method (Get Outstanding Invoices /
Paid Amount changes, so the tax row updates live) and on first save.
Saved entries keep the user's choice; unticking now removes the row.
Client script recomputes the row when the checkbox or category changes.