I assume you are on android as it is the platform with most such issues. If you are you should just switch back to the game after the page freezes after you press allow and it should work.
Is usually works, so I won't be touching it anytime soon. You should follow the steps I told you if you are on Android:
Open the game -> Click 'Authenticate' -> In the webpage click 'Allow' -> Switch back to the game even if the page doesn't load further
If you are on any other platform (Windows, Mac, Linux), you will need to tell me what you do and what exactly happens, as it hasn't been reported that it doesn't work there.