Change Author (From) email address on Email Messages via > Create New > Email

I need to change the "From" address to a generic email address for emails originating from Invoices.   One of our business units uses the baked-in Email Message form that you get when you use the " > Create New > Email" menu option while situated on an Invoice.

Looking at the HTML source of this form, I noticed that the "author" field had my employee internal Id in it.  So, I tried to change it to another value (an Id of a generic Employee record) via the browser console.  That didn't work.  I also wrote a User Event script against the Message record type.   I wrote beforeLoad, beforeSubmit, and afterSubmit functions and tried to change the Author field to the new value, but when the email gets sent, it still comes from my personal email address.