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I'm here to assist you with your query about a custom field. You do have an option to add a custom field to your invoices.
Let me show you how:
If you want to take custom templates on a different level, you have the option to create and upload your own in QuickBooks Online. See this article for the steps: Import custom form styles for invoices or estimates.
If you need anything else, don't hesitate to leave a comment below. I'll get back to you as soon as I can.