How to exclude VAT if client registers with VAT id?

Since in opencart is it possible to add custom field on registration form, and as as i know VAT number is not a default field I would like to know how can i add a VAD ID field ( not required ) and not charge VAT if client is inside a specific geo zone

There are plenty of extensions that add this, I suggest a search for a suitable one there first.

I made a search before posting
most of the extension seem to validate VAT but don't really explain how hat impact order total

https://www.opencart.com/index.php?rout ... 0exemption

i guess the only one could be the EU VAT Compliant, not sure why it appears multiple times in search result i notice that with other extension

My idea i will try is extra customer group, where customer must be aproved by admin (After EU VAT validation) - and extra TAX setting (w/o VAT) for this customer group. Disadvantage is that it cant be automatic proces, and Admin has validate VAT and then approve customer ...