Hi, I've never used pfSense but it looks like that you have one interface on the WAN site and one on the LAN.
In that case create a bridge@EWC topology for the guests, a wireless open/none service linked to this topology and connect the topology to the LAN port of the pfSense.
pfSense would act as the DHCP server for the wirless clients, as the box is also the default gw every traffic would pass thru it which would result that the initial traffic will trigger the captive portal.
So no big deal from the WLAN appliance point of view.
In that case you didn't even set the authentication to external captive portal... it's just a open/none SSID....