Hello Kuljeet,
It looks like while the format of the Authorization Request URL is correct, your redirect_uri does not match what you have set as the Redirect URI in the My Applications section of the V3 portal in your account.
In order to successfully redirect to the authorization page, those two values will need to match. Once the value is corrected, it should be able to redirect without issue.
https://v3.developer.constantcontact.com/login/index.html
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