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:

if (!in_array($_SERVER['HTTP_X_REAL_IP'], array('easycronIP1', 'easycronIP2', 'easycronIP3', ...))) {

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.

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.