How to trigger a hold for a specific invoice match type?

Summary:

A custom hold has been created and we wish to call it when matching a 2-way PO to an invoice.
The requirement is to trigger a hold or a message when a 2-way PO is matched to an invoice or when that invoice is validated.

Content (please ensure you mask any confidential information):

A custom hold has been created and we wish to call it when matching a 2-way PO to an invoice.The requirement is to trigger a hold or a message when a 2-way PO is matched to an invoice or when that invoice is validated.

Setups have been done on the BPM Tasks 'FinApHoldApproval' and 'FinApInvoiceApproval', in which we attempted to set a message on the invoice header. Please refer to screenshots. The outcome is not as expected.