Thank you for reaching out to Constant Contact's API Support.
Both of our API versions require an initial user interaction to login to the Constant Contact account and click the Allow button. Once that happens once you get an Access Token. The difference is when the Access Token expires. In our v2 API the token is not super secure in that it expires after multiple years. In our v3 API the token expires after 24 hours. When using our v3 API you also get a Refresh Token to refresh the Access Token when it expires, this does not require the user to interact.
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