How to automatically login to captive portals on OS X?
Just found this Disable Captive Network Support in OS X

Which mentions "To disable it, set this preference:"

sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.captive.control Active -boolean false


In case, you would ever want to remove this setting, you can do:

sudo defaults delete /Library/Preferences/SystemConfiguration/com.apple.captive.control Active


Then, as @Leon mentions, use the regular web browser to login. Or if you are on a network where that somehow does not work then you can still try and open the app manually ( located at /System/Library/CoreServices/Captive Network Assistant.app )