How can add VAT to my invoices?

You'll want to set up the tax first, info.

QuickBooks Online has the Tax feature which tracks your sales tax. You can also create your tax codes. Here's how to turn it on:

1. Click Taxes on the left pane.
2. Select Set up tax.
3. Set up the necessary details ,then select Next.
4. Follow the steps to finish and select Done.

Once done, you'll see the Tax Centre. You'll also see a Tax/GST column on your sales and purchases forms. When you create an invoice, you'll see the Tax column. Just select the appropriate VAT for the invoice created.

Check this article on applying sales tax to transactions: https://community.intuit.com/articles/1775733.

Stay in touch with me if you got more questions about sales tax.