I wanted to do the same thing for just my Subject line...I use the partial method from the EZ map template and Typed this in my template: Invoice [Invoice No.] from xxx for [customername]. It works for the invoice no where it will automatically insert the Invoice No but I can't get the customer name to autopopulate. So when I email...this is what it does generate. So maybe you could try and download the EZ map template and try to pull the field names of what you need and insert into the message.
Invoice 12345 from KLP Commercial, LLC for [customername]
