I just want to create a simple sign up form for new users to a specific contacts list using the API. I've looked over the documentation but it looks like to use the contact_data endpoint I need to get user authorization. How can I bypass this so I can retrieve the token to have access to the endpoint? I'm following the instructions here: https://v3.developer.constantcontact.com/api_guide/server_flow.html . But in the first step I'm supposed to create an authorization request. If this is a new user, how are they supposed to sign in authenticate?