Good day, galteecivil.
It's nice to see you here! Let me show you how to add the customer's ABN to the invoice. Please follow these steps:
Once done, let's customise the invoice to appear there.
I've added this link for your reference: Customise invoices, quotes, and sales receipts in QuickBooks Online.
Post again in the QuickBooks Community if you need more help. Stay safe!