Registering in an external website before login to the App

Hi,
we have built an App which aprroval has been rejected because we ask to enter some registration personal data that is neccesary for some material gathering during a social event. There is a data gathering agreement screen where the users accept that data gathering. It seems this is not valid for Apple because they say we break 5.1.1 policy about collecting non essential data for the core functionality...
we are considering to just require the user to enter his email in order to log in and we would supply a link to an external registration web page so that the users register themselves in that website, outside the App.
Is it possible to require to be registered in an external website to be able to use an App? would that be allowed by Apple policies?

Has anybody any similar experience with this kind of issue?

Thank you very much!
Registering in an external website before login to the App
 
 
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