I follow these steps to get access token using post man. for v3    1. Loginto https://app.constantcontact.com/pages/dma/portal/ 2. create app and generate secretcode 3. make below request through browser after changing client_id https://api.cc.email/v3/idfed?client_id=7391d41d-15f8-4767-a8e5-a43d1fa5f1bb&redirect_uri=https://localhost&response_type=code&scope=account_read+account_update+contact_data+campaign_data 4. Enable f12 for browser debug mode it will show the url in debug mode something like https://localhost?code=Btm-DfwTesRXTplmOCEGpBI6CTD_GTL1FPclEAEt 5. Open PostMan and select request type as Post and make the below request after changing the Basic Auth and code parameter Basic <client_id>:<secret_key>(base64encode) https://idfed.constantcontact.com/as/token.oauth2?code=GfqUoci56ZQF7jV9tFXVslgXRJjUzLDsYVPBcgDJ&redirect_uri=https://localhost&grant_type=authorization_code     Im getting this json error message. { "error_description": "unable to parse Authorization header javax.security.auth.login.LoginException: HTTP Basic username has invalid content com.pingidentity.common.security.InvalidInputException: Value for parameter \"Authorization\" is invalid. Value doesn't match regular expression: .*", "error": "invalid_client" }     ... View more