Here is the user guide for version 10.11. Please refer page 176 to "Configuring a Captive Portal on an AP".
With Bridged@AP, the only Captive Portal/Spash page integration method is Firewall Friendly External Captive Portal (FFECP). The Splash page/webserver needs to integrate with the FFECP protocol definition. Specification details on FFECP covered in the ExtremeWireless(TM) integration guide:
This implementation is already directly supported by some 3rd party vendors such as PurpleWifi.
Note: If using ExtremeCloud(TM), as of 3.21.01, Splash page facility is directly integrated in the Network services definition.