Trying to bring access token using approval code, client id, etc

I'm using C# to implement youtube login in my application and I was wondering why it is keep returning 'bad request' message whenever I request for access tokenI've got client id, client secret, redirect uri, etc to access but it keeps returning me bad request.Please tell me whats wrong with my code

            try            {                string clientId = "~~~";                string clientSecret = "~~~";                string redirectUri = "urn:ietf:wg:oauth:2.0:oob";                string tokenEndpoint = "https://oauth2.googleapis.com/token";                HttpClient client = new HttpClient();                var content = new FormUrlEncodedContent(new[]                {            new KeyValuePair<string, string>("grant_type", "authorization_code"),            new KeyValuePair<string, string>("code", code),            new KeyValuePair<string, string>("redirect_uri", redirectUri),            new KeyValuePair<string, string>("response_type", "token"),            new KeyValuePair<string, string>("scope", "https://www.googleapis.com/auth/youtube"),            new KeyValuePair<string, string>("client_id", clientId),            new KeyValuePair<string, string>("client_secret", clientSecret)        });                HttpResponseMessage response = await client.PostAsync(tokenEndpoint, content);