How to create a standard 'message' on a custom invoice?

Question

I have created a custom invoice and would like to include a standard message in the message section of all the invoices but it doesn't seem to allow be to type in anything.  Does anyone know how to include a standard message in the message section at the bottom left of the invoices?

Greg_the_Geek · Accepted Answer

Click the down arrow by Customer message: and select New/Edit.

Greg_the_Geek · Accepted Answer

Not in the Design Invoice Forms, from the Invoice itself.