AutoAuth is disabled by default so to enable it for WHMCS v6.0 to v7.10, you will need to add the following line to your WHMCS configuration.php file to define an AutoAuth key. The value just needs to be a random sequence of letters and numbers:

$autoauthkey = "abcXYZ123";

This stape is not needed in WHMCS v8.0 and latest