I understand. I've changed the configuration and now the VAT ID is showing, and you can enter your VAT ID.
Please note that, the VAT ID might not be available in your location. Here is a quote from FastSpring docs:
Customers in some locations can enter their VAT ID or GST ID during the checkout process if they are exempt from payment of the tax. With a valid ID, FastSpring does not collect tax, and the ID entered by the customer appears on the invoice we provide upon purchase completion.
If a customer forgets or is unable to enter a VAT ID or GST ID at the time of the purchase, they can submit a copy of their valid VAT ID or GST ID at https://questionacharge.com along with their order reference/order ID within 30 days of the purchase. FastSpring will issue a credit to the original payment method.