Hello Charlotte Drew:
Welcome to the Microsoft Community. I am glad to help you with this question :)
You can add your VAT ID in the Microsoft 365 admin center. However, the steps to add a VAT ID depend on the type of billing account that you have. Find out what type of billing account you have.
1.Add your tax id for your MCA billing account
- Go to the Microsoft 365 admin center.
- Select the Navigation menu, then select Billing > Billing accounts.
- On the Billing accounts page, select the billing account.
- On the billing account details page, select Add tax ID.
- Enter your Tax id, then select Save.
NOTE: Not all regions support self-serve tax id management.
2.Add your VAT ID, GSTIN, or PAN for your MOSA billing account
- Go to the Microsoft 365 admin center.
- If you're using the Simplified view, select Billing.
- If you're using the Dashboard view, go to the Billing > Your products page.
- Select the subscription that you want to update.
- On the subscription details page, in the Service usage address section, select Edit service usage address.
- In the Edit service usage address pane, in the VAT, GST, or PAN number box, enter your ID.
- Select Save.
Reference link: Tax information for Microsoft 365 for business products and services | Microsoft Learn
Thank you so much for your precious time.
Best regards,
Doris V - MSFT | Microsoft Community Support Specialist