See this section in the README, that should clear it up for you
https://github.com/ueberauth/ueberauth_facebook#calling

If that doesn’t help, perhaps you could be more specific? Do you have a code example or an error message?

Hey @lukerollans, thanks for the reply. I actually got the idea of using /auth/facebook?scope=email,public_profile&auth_type=rerequest from the same source. I’ll try to be more specific using the phoenix guardian example (https://github.com/hassox/phoenix_guardian/blob/ueberauth-guardian/web/auth/user_from_auth.ex#L37).

Let’s add some basic code to check for an email.

defp validate_auth_for_registration(auth) do
  case auth.info.email do
    nil -> {:error, :email_is_null}
    "" -> {:error, :password_is_empty}
    _ -> :ok
  end
end

With this example, the authentication will get an {:error, :email_is_null} when the user doesn’t give permission for their email as expected. However, if the user realizes that the application needs his/her email and decides to try again, the user will get the error message without the permission prompt. I just found that you need to remove the app from https://www.facebook.com/settings?tab=applications to try again.

------- Update -------

It seems like this will do the trick.

  {:error, :email_is_null} ->
    conn
    |> redirect(to: "/auth/facebook?scope=email&auth_type=rerequest")