Is there a way to add a "message" in the Paypal submission?

Is there a way to add a "message" in the Paypal submission so that the recipient (our accountant) can tell who/what the payment is for easily?

I am not sure if I understand your query correctly. You may pass a Text box field to PayPal payment by using the option 'Fetch Data from a Text field' in the Additional Gateway settings.

The field data sent to the PayPal shall be displaying in the payment description that might help. Let us know if you are referring something different. We will be happy to help. 

This is exactly what I need! However, I'm not seeing this in the settings.
Where can I find the Fetch Data settings?
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You may find the option in the Additional Gateway settings in the payment field wizard. Please see the screenshot below:

Thanks!