Hook verify custom field

[Clebson]

Hello

 

I saw on some sites that have WHMCS, some hook that checks for custom fields with the database.

 

Works the same way that the email field, ie, it checks if the email already exists in the database. With only difference that the check occurs after typing the characters in the field.

 

Someone has some hook as an example?

 

Thank you in advance who can help me.