Which side would be your iperf server?
Is the Fortigate configured with a public IP address?
If you plan on using a device connected to FGT as the iperf server, you can use VIP on the FGT to allow the traffic. Iperf allows you to use your own port (either TCP / UDP), set port 8080 as the listening port for iperf server and configure VIP.
If the Fortigate is not configured with a public IP, then it might get tricky. You can configure DynDNS and use the DNS name for the iperf. Same FQDN can be used in VIP as external IP.