Hello user742094,   Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.   Each of our available OAuth2 authorization flows requires the use of a browser window for a user to grant access to an application for their Constant Contact account through a permission request screen, however, you should only need to do this once as after access has been granted, your application can maintain account access through the use of refresh tokens.   If you don’t want to use redirects for authorization, I would recommend looking into our device flow, which doesn’t use redirects or the client secret. I’m including documentation for this authorization flow below.   OAuth2 Device Flow: https://developer.constantcontact.com/api_guide/device_flow.html   Please have a look and let us know if you have any other questions!   Regards, ... View more