Hi Anupam ,

The code is like this

string sfdcConsumerKey = "3MVG9ZL0ppGP5UrB7LcMsVM21rMIyJwaCTeWpe._4T5k.lWlJTt56kcJrdOmGUETdwhUnRN.jK";

        string sfdcConsumerSecret = "8760078wwww71430600114";

        string redirectURL = "https://login.salesforce.com/services/oauth2/callback";
        string sfdcToken = "BBrUsrwl2srrr0kNhrUrrrrr9DHjir";
        string sfdcUserName = "jdddd@gmail.com";
        string sfdcPassword = "xxxxxxx";

        string loginPassword = sfdcPassword + sfdcToken;
        string URI = "https://login.salesforce.com/services/oauth2/token";

        StringBuilder body = new StringBuilder();
        body.Append("grant_type=password&");
        body.Append("client_id=" + sfdcConsumerKey + "&");
        body.Append("client_secret=" + sfdcConsumerSecret + "&");
        body.Append("username=" + sfdcUserName + "&");
        body.Append("password = " + loginPassword);

        HttpClient authClient = new HttpClient();

        HttpContent content = new FormUrlEncodedContent(new Dictionary<string, string>
  {
     {"grant_type","password"},
     {"client_id",sfdcConsumerKey},
     {"client_secret",sfdcConsumerSecret},
     {"username",sfdcUserName},
     {"password",loginPassword}
   }
        );

        HttpResponseMessage message = authClient.PostAsync("https://login.salesforce.com/services/oauth2/token", content).Result;

        string responseString = message.Content.ReadAsStringAsync().Result;

        JObject obj = JObject.Parse(responseString);
        string oauthToken = (string)obj["access_token"];
        string serviceUrl = (string)obj["instance_url"];


        /// posting the order data
        /// 

        string xmldata = @"<order><attributes><type>Order</type></attributes><EffectiveDate>2015-04-11</EffectiveDate><Status>Draft</Status><billingCity>SFO-Inside-OrderEntity-1</billingCity><accountId>00128000002l6q5AAA</accountId><Pricebook2Id>01s28000000uD8IAAU</Pricebook2Id><OrderItems><records><attributes><type>OrderItem</type></attributes><PricebookEntryId>01u28000000eeirAAA</PricebookEntryId><quantity>1</quantity><UnitPrice>15.99</UnitPrice></records></OrderItems></order>";

        var elements = XElement.Parse(xmldata);


        xmldata = elements.ToString();
        HttpClient createClient = new HttpClient();
        content = new StringContent(xmldata, Encoding.UTF8, "application/xml");
        string uri = serviceUrl + "/services/data/v30.0/commerce/sale/order";
        HttpRequestMessage postrequest = null;
        postrequest = new HttpRequestMessage(HttpMethod.Post, uri);


        postrequest.Headers.Add("Authorization", "Bearer " + oauthToken);
        postrequest.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));
        postrequest.Content = content;
        HttpResponseMessage postresponse = createClient.SendAsync(postrequest).Result;
        string responsedata = postresponse.Content.ReadAsStringAsync().Result;

Thanks and Regards
Utpal Maity