I want to add the TRN number to the header of the invoice

No worries. I’m here to show you where to add the company TRN number to the sales invoice for QuickBooks Online (QBO), @h.shakka.

You can easily customize an invoice and add your company registration number by going to the Custom Form Styles section in QBO.

Here's how:

1. Within your QuickBooks account, click the Gear icon.
2. Select Custom Form Styles under Your Company.
3. Select the Invoice template you're using.
4. From the Actions column, click Edit.
5. Click the Content tab.
6. Select the Header part on the sample invoice. 
7. Put a check mark on the Company registration number and enter the company tax number in the CRN field.
8. Click Done.

For more comprehensive information on personalizing your sales forms, I recommend reading through this article. It offers in-depth insights and step-by-step instructions to help you customize your sales documents and meet your specific needs: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Additionally, I've added an article that provides valuable insights on effectively managing your sales forms in QuickBooks. These resources will guide you through various features and techniques, helping you streamline your invoicing process and enhance your overall business efficiency. Add, customise, or remove logos on sales forms.

If you have any further concerns or questions about managing your invoices in QuickBooks Online, please don’t hesitate to reply to this post. I’m here to provide assistance and support you every step of the way, @h.shakka.