OSTICKET is a great open source helpdesk solution that help much IT guys all over the world have a tricky part when it come to sending notification by mail with the new tickets.
So I made a setup to the system and set the crontab line then tested it,
* * * * * root php -f /var/www/helpdesk.yourcompanydomainname.com/public_html/api/cron.php
Below explanation for each part of the command:
* * * * * means that the system well check every one minute.
root means that you chooses to run the command with root privileges
php is the place where your php engine is (and it is smart enough to know the location when you say php)
-f I do not remember the reason for it.
/var/www/helpdesk.yourcompanydomainname.com/public_html/api/cron.php is the location where the cron.php is located. You can use locate from the command line if you have ability to open terminal