Duplicate check for tax number

Hello,

I have a question guys, When I want to create e new Customer there is "Check For Duplicate" button.

We had inserted two custom filed that is Tax Number 1 and VAT. My question is that how can I change "Check for Duplicates! button to check also duplication of Tax number 1 and VAT?

I wanted to create a script in BOE, but there is just Event before save, I dont know how can I do it for this?

also in the EventHandler of UI I can not modify this event handler.

I would be appreciate if you can help me about this?

Thanks