I appreciate you for posting this here in the Community, @Sandy312. I'd be happy to share some insights about this, along with a workaround to be able to add another section for your invoice number in your sales form in QuickBooks Online (QBO).

First and foremost, I understand the importance of being able to have an extra box in your sales form. Here in QuickBooks, you can use the Custom fields to add box for your invoice number when creating a quote. You can create up to 3 custom fields and add them to your quote in QBO. What's good is that if you're using QuickBooks Online Advanced, you can create up to 12 custom fields. I'd be happy to outline the steps below so you can get through this and add an extra box for your invoice number. 

To begin, here's how:

1. Go to the Gear icon, then select Custom fields.
2. Select Add field.
3. Enter a name in the Name field.
4. Select the All Sales forms checkbox.
5. If you want the custom field to appear on printed and delivered forms, turn on the Print on form switch. Otherwise, it will only be visible in QuickBooks.
6. Select Save.

Feel free to visit this article for further guidelines on using the custom fields: How to add custom fields to sales forms and purchase orders.

Refer to the screenshot below to see the changes once you use custom fields when you create a quote in QBO.

In addition, I've got you this article in case you'd like to make personalisation on your sales forms: Add, customise, or remove logos on sales forms.

You can always count on me if you have any additional QuickBooks-related concerns. Feel free to click the REPLY button below. This way, I'll be notified, and be able to lend you a hand. Take care!