Adding an Invoice Comment

I'm setting up our company for the first time and working on our invoice layout. I need a field at the top where I can write unique comments to the customer. I found a field called "Invoice Comment" that I thought would work for this, but when I go to make an sales invoice I can't find a place to write the comment.

The closest I've found is adding a note that will appear in the section with the line items, but that's not what I need.

Does anyone know how to add the data that populates the "Invoice Comment" field?

Please and thank you!