adding vat to company details when creating an invoice

I can help you with that, EE102378025.

Your VAT registration number will automatically populate on your invoice's header section once you set up your VAT in QBO and your transaction uses VAT code. You'll want to make sure that you already have your VAT Registration No entered, here's how:

1. Go to the Taxes menu.
2. Select the VAT tab.
3. Click the Edit VAT drop-down menu, then select Edit settings.
4. Enter the VAT registration number.
5. Hit Save. 

To help you personalize your invoice template and add specific information to your sales forms, you can go read this article: Customize invoices in QuickBooks Online. 

Let me know in the comment section if you have any other concerns or questions about VAT. I'm always here to assist.