External Captive Portal
When an external access controller, such as a captive portal appliance or a Universal Access Method (UAM) redirector is required to handle subscriber login, OpenWiFi now supports ability to have a native implementation that is equivalent of CoovaChili.
As always first define a captive service as part of the interface -> ssids.
The captive service must have the auth-mode set to "uam"
For further details on how to configure with a external service please refer to this.