in my case , i would like to let users(guests) self registration in our fortiauthenticator , so i would like to except https://x.x.x.x/registration (x.x.x.x fortiauthentificator address ) from captive portal , like that users can access to registration page.
i have create a policy , but i back to captive portal all times .
on 5.2 FortiOS set FortiAuthenticator IP as firewall address object and then go to interface setting and right below the captive portal is possibility to set "Exempt List" .. add FortiAuthenticator's IP address object into it. It will exclude that address from the portal.
i have found the solution in older version ( v5.0,build0310 ) . the SSID must be Open , and you create a user identity policy and other policy to allow access with http/https to the self registration page .