Hello user2793833, 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. The Redirect URI in your Authorization Request URL must exactly match one of the Redirect URIs specified in your key's settings. To access your key's Redirect URIs: Navigate to https://app.constantcontact.com/pages/dma/portal and click the "Edit" button to the right of the key you are using. OAuth2 Implicit Flow https://v3.developer.constantcontact.com/api_guide/client_flow.html Please have a look and let us know if you have any other questions!
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