How to post data to rest web api and get response

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ASP.NET

REST

WebAPI

Hi developers I am consuming web api first time .
This is my json object to authenticate a user . how can I pass this asp.net to call web api for getting authenticate token please help me


POST /v2.0/tokens HTTP/1.1
User-Agent: curl/7.21.0 (x86_64-pc-linux-gnu) libcurl/7.21.0 OpenSSL/0.9.8o zlib/1.2.3.4 libidn/1.15 libssh2/1.2.6
Host: identity.api.rackspacecloud.com
Accept: application/json
Content-Type: application/json
Content-Length: 54

{
"auth":
{
"RAX-KSKEY:apiKeyCredentials":
{
"username": (1)"jsmith",
"apiKey": (2)"aaaaa-bbbbb-ccccc-12345678"
}
}
}

My requirement is that I want to Post this json object to server and get the response from server but I am getting exception of bad request 400. what is the cause of this? I asked to google about this many times and found lots of solution and tried all those but still I am getting same error
I am using this code in asp.net

            // Create a request using a URL that can receive a post. 
            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create("https://identity.api.rackspacecloud.com/v2.0/tokens");
          
            request.Method = "POST";
     
            string postData = "{ \"auth\":{\"RAX-KSKEY:apiKeyCredentials\":{\"username\":\"jsmith\" ,\"apikey\":\"aaaaa-bbbbb-ccccc-12345678\" }} }";
          
           
            var byteArray = Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(postData));
            ASCIIEncoding encoding = new ASCIIEncoding();
            byte[] data = encoding.GetBytes(postData);

           request.Accept = "application/json";

          
           request.ContentType= "application/json";
        
  
             request.ContentLength = data.Length;

          
           Stream dataStream = request.GetRequestStream();
    
           dataStream.Write(data, 0, data.Length);
     dataStream.Close();
            
  Getting exception in this line
        HttpWebResponse response = (HttpWebResponse)request.GetResponse();
             // Display the status.
             Console.WriteLine(((HttpWebResponse)response).StatusDescription);
             // Get the stream containing content returned by the server.
           dataStream = response.GetResponseStream();
             // Open the stream using a StreamReader for easy access.
            StreamReader reader = new StreamReader(dataStream);
             // Read the content.
           string responseFromServer = reader.ReadToEnd();
             // Display the content.
            Console.WriteLine(responseFromServer);
             // Clean up the streams.
             reader.Close();
            dataStream.Close();
             response.Close();

            
        }

please give me some solution .......