Adding Billing Notes to an Invoice - the option is there but not sure on how you add the text!

How do I add billing notes text to an invoice?

As you can see, when you go to view the invoice it says in red text “BILLING NOTES” so the option is there to add text, but I cannot find the place where I can add the text?

Where do I find this?

Hope this is clear.

We are not seeing the same. I would therefor guess that this is possibly a custom field?

Thank you! I shall look into the custom fields.