Changing the email subject for billing document

A request is to update the email subject from the standard

INVOICE: 123456789 Customer Name

to

MyCompany Invoice: 123456789 Customer Name

I am looking for configuration to accomplish this task. The output determination configuration doesn't have an area to make a simple change. How can this be accomplished? I tried to make some changes in he "Mail title and texts" but was unsuccessful.

Other postings were not helpful to find the correct answer.

Thank you,