The text is on each invoice, it’s not global. So you put the token on the invoice template then when you add a message onto the invoice, it will appear on the customer’s invoice.
Take a look at the attached screen shot. You will see there is a place to add text for the Invoice disclaimer.
I am expecting to see a similar box for the Invoice message tag- (2nd screen shot). I know I can add text to the bottom of the Invoice, but I would like to do it in a tag. Is this not possible?
You are trying to use a tag that’s not its intended purpose. As I said, this tag/token is used for a single message per invoice. You add that to your template, then any time you fill in the message box, it will show up on the customer’s invoice. If you are needing to add static text to every invoice, then you do that on the invoice template or in the disclaimer.