Hook Verify custom field in the client register

[Clebson]

Hii,

 

I have created some custom fields in my WHMCS to be displayed on the customer's registration page.

 

One of these fields the client must inform the single document of the person.

 

I want to know some way to check if there is already a record in the database according to what the customer inform in the field. How can I do this for a custom field?

 

I noticed he does this check for the email.

 

I know it can be done via HOOK, I even tried to create but I could not, I do not know exactly how it works.

 

Does anyone have an example or do know of any simple way to do this verification?

 

I want to do the verification using the "onblur" event in the input field of the registration page.

 

Thank you very much in advance!