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
Please ensure `openprovider_additional_fields()` function has been correctly configured in the file resources/domains/additionalfields.php according to the instructions in this article.
If the file
<your WHMCS directory>/resources/domains/additionalfields.phpthen add the example version from GitHub. 
Retry domain registration.