Your integration needs to connect to a Constant Contact account in order to know where to add the email address. The way that your integration connects to the account is through oAuth; the end result of oAuth is to get an Access Token.
The API Key allows your integration to talk to our system. The Access Token tells our system which Constant Contact account to send/retrieve data from.
It sounds like you are making a type of sign-up form. In that situation all of the authentication would happen behind the scenes. The "client/people" that are signing up do not have to authenticate. Your integration aka script is what needs to authenticate. In the v2 API you did this by using the API Key and Access Token which you also may know as a Bearer number.
It may be beneficial at this point to have you email us at email@example.com so you can provide us your specific information and we can give more detailed specifics that should not be shared publicly.