WHMCS allows creation of a ticket via email. I only accept emails from registered users.
1. I set whmcs/crons/pipe.php permissions to 755 (instead of 644).
2. In the whmcs/crons config file, I set $whmcspath.
$whmcspath = '/home/username/public_html/whmcs/';
3. I set up email forward. The advantage of using email forward is it does not require a cron job to run every few minutes, looking for an email message.