It looks like public API doesn't do too much for you. It supress the message, but a user should manually launch another app to login.
However, there is a special entitlement "com.apple.developer.CaptiveNetworkPlugin" which gives you possibility to use bunch of private API's in CaptiveNetwork framework to build a real captive plugin. Some companies work with Apple to do this (https://devforums.apple.com/message/792112#792112). However, it's not a broadly available API.
BTW. Here is some additional information from a guy who reverse engineered one of such apps: http://kalapun.com/posts/reverse-engineering-ios-app/