The file you will be editing for the forwarding is located on whatever webserver your domain is on. It is httpd.conf most likely (if you are on a CentOS server, for Debian based systems you need to create a file in the sites-available folder). It's probably located in /etc/apache2. Use some obscure port number.
More info on that: http://httpd.apache.org/docs/2.2/vhosts/examples.html(http://httpd.apache.org/docs/2.2/vhosts/examples.html)
Opening the Port, Preferred Method:
You then need to make sure that your firewall forwards this obscure port to internal port 80 of your NAS's IP.
Example:
WAN IP - 169.58.67.27
Synology LAN IP - 10.0.1.5
Obscure Port picked: 9504
Firewall rule would be: 169.58.67.27 -> 10.0.1.5
Alternatively:
Add a new website via the Control Panel -> Web Services -> Virtual Host area and add a new virtual host with a port number other than 80. Copy your osTicket install there (copy all files). Forward whatever port you used inside your firewall to your Synology box.
Again this really doesn't make much sense to set things up this way instead of just putting osTicket on a real webserver.