This is discussed at the wiki.
source: http://osticket.com/wiki/POP3/IMAP_Settings(http://osticket.com/wiki/POP3/IMAP_Settings)
Recurring tasks scheduler (Cron Job)
This is the most convenient method if your hosting provider allows scheduling recurring tasks via crontab on *nix or "Scheduled Tasks" on a Windows server. Please refer to your hosting manual or contact your hosting provider for instructions. The PHP CLI executable is called to run cron.php.
Add the following entry to cron file normally in /etc/crontab in *nix systems and adjust the time and paths accordingly.
*/5 * * * * nobody /path/to/php /path/to/api/cron.php
For windows users in "Scheduled Tasks" add "c:\php\bin\php.exe c:\website\osticket\api\cron.php"
Recurring tasks scheduler (host's custom task scheduler)
Some hosts do not allow adding cron jobs, and instead only allow you to probe scripts located at a publicly accessible URL. In this case, you will schedule the task using your webhost's scheduling interface and http://domain/path/to/osticket/api/cron.php(http://domain/path/to/osticket/api/cron.php).
To prevent unauthorized execution of scripts from the outside, Settings => API has rules for allowing external access based on originating IP and an API "passphrase", which is actually the User Agent string of the agent trying to access cron.php on your web server.
If the User Agent string is not changed to the API key, the request will be denied, email will not be polled, and API error - code #77: Unknown remote host or invalid API key will appear in your system logs under the osTicket dashboard.
If your host allows you to modify their schedulers User Agent string, then change that to the API key and add your webhost scheduler's IP to Settings => API => API Keys.
External triggering using wget
First add an API key and IP to Settings => API => API Keys.
Enter the IP of the host that the wget will run from.
Enter 3 separate words into the passphrase box (these are used in conjunction with the IP to generate hashes)
Click submit
A hash will be generated
Then, setup a cron job to run wget */5 * * * * nobody wget -q -O /dev/null --user-agent= http:///api/cron.php