How to pass value from ActiveForm validation to JavaScript?

Question

Yii

How to return a value to JavaScript from attribute validation in ActiveForm so that it can be processed?

public function validateCoupon($attribute)
{
    return 0.2;
}

As I understand that I need to dig somewhere here, but I can not find where exactly.

$('#test-form').on('afterValidateAttribute', function (event, attribute, messages) {
    console.log(messages);
});

Or is it possible in some other way? You need to process the coupon on the server and return the value (discount) to JavaScript in order to change the price, etc. Naturally, it’s stupid to do this purely within the framework of JS, since the code will be visible, here it is necessary on the server.