string result = ""; 
string strPost = 
"x_login=YOUR-LOG-IN-ID&x_tran_key=YOUR-TRANSACTION-KEY&x_method=CC&x_type=AUTH_CAPTURE&x_amount=1.00&x_delim_data=TRUE&x_delim_char=|&x_relay_response=FALSE&x_card_num=4111111111111111&x_exp_date=052005&x_test_request=TRUE&x_version=3.1"; 




// Create a new request to the mentioned URL. 
HttpWebRequest objRequest = 
(HttpWebRequest)WebRequest.Create("https://certification.authorize.net/gateway/transact.dll"); 


// objRequest.Proxy= new WebProxy("8.3.8.101"); 
objRequest.Method = "POST"; 
objRequest.ContentLength = strPost.Length; 
objRequest.ContentType = "application/x-www-form-urlencoded"; 

string error=""; 
StreamWriter myWriter = null; 
// try 
// { 
myWriter = new StreamWriter(objRequest.GetRequestStream()); 
myWriter.Write(strPost); 
// } 
// catch (Exception ex) 
// { 
// error=ex.Message; 
// } 
// finally 
// { 
myWriter.Close(); 
// }