For someone who is looking for this question as a user, not a developer, there are several applications on Google Play that will help you as a user. For developers: some of them also offer APIs for use in other applications. Some examples:
- WiFi Login
- Logging In AfterConnect Online
- Captivate (shows a login screen, not an automatic login)
And to get around the latest version of Android that disables Wi-Fi when it detects a portable portal, try this app: Wifi Portal Connect. Then it will work together with one of the applications proposed above for reconnecting to Wi-Fi, and then other applications will be able to automatically log into the system.
Each individual portal is different, so keep that in mind, and not about application issues if it doesn't work, but applications become more flexible and automatically detect systems, so try a few.
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