I'm not familiar with Fortigate, but you would set a syslog destination as the IP and port that you have configured in Logstash. https://help.fortinet.com/fadc/4-5-1/olh/Content/FortiADC/handbook/log_remote.htm
In the example I gave, I'm using UDP port 5515. I personally like to test just using the stdout output plugin. It writes the log entries to the console - assuming you're launching Logststash from the console. https://www.elastic.co/guide/en/logstash/current/plugins-outputs-stdout.html