Thanks for posting your question on Community.
If you have already turned on the VAT in QuickBooks Online if you go to the Taxes tab and select Edit VAT then Edit settings you will then be able to enter your VAT number. If you have already entered the number this will not show on the invoice when on the preview section of the invoices, but this will show when sent with an Invoice including VAT.
Please let us know if there is anything else which we can help with regards to QuickBooks Online.
