How to add the invoice no into the title of mail messages for ERS Invoices?

How can I add the invoice number into the title of a mail message generated from an ERS Invoice ?

I know how to do it for a message from a Purchase Order -

In the definition of the Output Type, in the section "Replacement of text symbols", assign program SAPMM06E and Form routine TEXT_SYMBOL_REPLACE, and define the title as in the conditions "Purchase Order &EKKO-EBELN&". This results in a a title "Purchase order 4500012345" on the e-mail message.

It looks as though I should be able to do something similar for the ERS invoice, in MM > Logistics Invoice Verification > Message Determination > Maintain Messages Types, and select ERS output type.

Assigning the same program SAPMM06E for the ERS output type doesn't work, which is to be expected because the program does not process invoices.

I can't find a suitable program to use instead of SAPMM06E - is there a program for ERS invoices which includes TEXT_SYMBOL_REPLACE ? Or should I use a different form routine and program?

(Also, in MM Logistics Invoice Verification > Message Determination > Maintain Conditions - I have defined the title in the conditions as "Self Billing Document &RBKP-BELNR&" but "Self Billing Document" appears as the title on the output, without the document number.)

Note that this relates to Self-Billing Invoices that are sent to vendors in Purchasing. I am NOT looking for a solution for Customer Invoices in Sales.

Richard Thomson