# How to add a VAT customer field in WHMCS

## Question

Some domain registries (e.g. DNS PT, ITNIC) require submitting a VAT number when registering domains under company contact handles. WHMCS doesn't include this data field by default, therefore domain registration may fail with the "**Invalid VAT number**" error.

How to provide a VAT number during domain registration?

## Answer

1. Please ensure `***openprovider\_additional\_fields()***` <span>function has been correctly</span> configured in the file **resources/domains/additionalfields.php** according to the instructions in [this article](https://openprovider.help/books/api-integrations/page/whmcs-domain-module-install-and-configure-openprovider-domain-registrar-module).
2. <span>If the file</span>`<your WHMCS directory>/resources/domains/additionalfields.php`<span>then add the example version from </span>[GitHub](https://github.com/openprovider/Openprovider-WHMCS-domains/blob/master/resources/domains/additionalfields.php).
3. Retry domain registration.