Answer:

There are a few ways to achieve this.

1) You may only allow requests from our IPs in your cron job script.

A quick example for PHP script is:

<?php
if (!in_array($_SERVER['REMOTE_ADDR'], array('easycronIP1', 'easycronIP2', 'easycronIP3', ...))) {
    die();
}
?> 

By adding above code to the beginning of your cron job PHP script, no one will be able to run your cron job script except EasyCron. For other programming languages, you could easily write some equivalent codes.

Please note that if you're using any proxy in front of your website server, above PHP code will not work. Instead, you may need to firstly change the web server setting in your proxy. For example, in a nginx reverse proxy, add

proxy_set_header X-Real-IP $remote_addr;

as described at here, and then use the following code to protect your cron job script:

<?php
if (!in_array($_SERVER['HTTP_X_REAL_IP'], array('easycronIP1', 'easycronIP2', 'easycronIP3', ...))) {
    die();
}
?> 

If you don't have control to the reverse proxy, please consult the proxy provider for what variable you could use to get the client IP.

2) Adding "HTTP basic authentication" to your cron job URL. There are many tutorials online that you could follow to create password protected URL such as this and this.