Hi leanne-farrell-t,
Thanks for posting in the Community! As of the moment, we don't have an option to add a custom field to sales form email subject or header. As a workaround, you can manually change the subject on the invoice before you send it.
I'd recommend you log this feedback in QuickBooks Online which goes directly to our product team for review and to help improve your experience in QuickBooks Online. Here's how to send a feedback.
Feel free to post again if you have any other questions. I'm here to help.
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