WP_Remote_Get Not working

The very concept of HTTP API is to make sure transport will be done. It basically uses 5 different transport methods and it chooses the best one according to your server config. So it’s unlikely a compatibility issue with wp_remote_get() and your server.

Plus if WP is not loaded, adding an action won’t help, it will fail the same with undefined function error but this time on add_action.

So basically you’re missing WordPress, for test purpose you could do this (assuming your file is at the root of WP installation ) :

<?php 
require_once( 'wp-load.php' );
$response = wp_remote_get( 'http://www.domain.com/mytest.php' );
print $response ['body']; ?>