Port Forwarding using DDNS

I'm new to Fortigate firewall, I've already set up port forwarding using 100D, what I'm  looking for is the ability to use DDNS to allow external user to access an internal web server (or what ever) using a domain name DOM1.fortiddns.com.

The thing is in port forwarding there is no possibilities other than "hard-coded" IP address.

Could someone point me out how to achieve this requirement ?