How use zadarma webhook

Hello, I am trying to use zadarma webhook, but I have problem with the set-up.

I suspect that the problem is about data validation.

Zadarma says this:

In order for the system to accept your request, please add the verification code at the beginning of the script.

PHP example:

<?php if (isset($_GET['zd_echo'])) exit($_GET['zd_echo']); ?>

How can I set up this webhook properly?

this is zadarma api guide: