How can I make my vat number appear on outgoing invoices?

Hi, niall-delaney1-g. 

Generally, VAT Number will automatically populate on your invoice's if you set up it succesfully. 

1. Go to the Gear icon ⚙ and select Custom Form Styles.
2. Click the New Style drop-down button.
3. Choose Invoice.
4. Go to the Content tab. Then, click the header part.
5. Hit Done.

Aside from that, we can also create our own template using an MS Word document. Then, import it into your QBO account. This gives you more flexibility when it comes to the details that you want to be part of your invoices and other sales transactions. Follow the steps in this article for more information: Import Custom Form Styles for Invoices or Estimates. 

On the other hand, we can get more tips while personalising your invoices and other forms from this article: Customise invoices, estimates, and sales receipts in QuickBooks Online.

If you have any other questions about your invoices and other forms, let me know by adding a comment below. I'm always here to help. Keep safe!