How do I put my abn in my head letter

Hello there, @2ezlouisy-lw-gma.

You can add your  Australian Business Number or ABN in your sales forms using the QuickBooks Online Custom Form Style Feature. I'd be happy to show you how:

1. Go to the Settings, select Custom Form Styles under Your Company column.
2. Click the New Style button and choose if it's an Invoice, Quote, or Sales receipt transaction.
3. Select the Content tab and click on the Header section of the transaction to reveal the details.
4. Check the box for ABN and enter your 11 digit ABN number.
5. Select Done. 

I've also  included some screenshots for your additional reference:

To know more about this topic, you can check out this article: Customise invoices, quotes, and sales receipts in QuickBooks Online.

I've added this article for your future task whenever you'd like to bring a finished sales template to your QuickBooks Online company: Import custom form styles for invoices or estimates.

Feel free to comment below if you have other questions. I'd be happy to answer them. Take care always.