How to add parameters into a WebRequest?

I need to call a method from a webservice, so I've written this code:

 private string urlPath = "http://xxx.xxx.xxx/manager/";
 string request = urlPath + "index.php/org/get_org_form";
 WebRequest webRequest = WebRequest.Create(request);
 webRequest.Method = "POST";
 webRequest.ContentType = "application/x-www-form-urlencoded";
 webRequest.
 webRequest.ContentLength = 0;
 WebResponse webResponse = webRequest.GetResponse();

But this method requires some parameters, as following:

Post data:

_username:'API USER',         // api authentication username

_password:'API PASSWORD',     // api authentication password

How can I add these parameters into this Webrequest?

Thanks in advance.

Use stream to write content to webrequest

string data = "username=<value>&password=<value>"; //replace <value>
byte[] dataStream = Encoding.UTF8.GetBytes(data);
private string urlPath = "http://xxx.xxx.xxx/manager/";
string request = urlPath + "index.php/org/get_org_form";
WebRequest webRequest = WebRequest.Create(request);
webRequest.Method = "POST";
webRequest.ContentType = "application/x-www-form-urlencoded";
webRequest.ContentLength = dataStream.Length;  
Stream newStream=webRequest.GetRequestStream();
// Send the data.
newStream.Write(dataStream,0,dataStream.Length);
newStream.Close();
WebResponse webResponse = webRequest.GetResponse();  

If these are the parameters of url-string then you need to add them through '?' and '&' chars, for example http://example.com/index.aspx?username=Api_user&password=Api_password.

If these are the parameters of POST request, then you need to create POST data and write it to request stream. Here is sample method:

private static string doRequestWithBytesPostData(string requestUri, string method, byte[] postData,
                                        CookieContainer cookieContainer,
                                        string userAgent, string acceptHeaderString,
                                        string referer,
                                        string contentType, out string responseUri)
        {
            var result = "";
            if (!string.IsNullOrEmpty(requestUri))
            {
                var request = WebRequest.Create(requestUri) as HttpWebRequest;
                if (request != null)
                {
                    request.KeepAlive = true;
                    var cachePolicy = new RequestCachePolicy(RequestCacheLevel.BypassCache);
                    request.CachePolicy = cachePolicy;
                    request.Expect = null;
                    if (!string.IsNullOrEmpty(method))
                        request.Method = method;
                    if (!string.IsNullOrEmpty(acceptHeaderString))
                        request.Accept = acceptHeaderString;
                    if (!string.IsNullOrEmpty(referer))
                        request.Referer = referer;
                    if (!string.IsNullOrEmpty(contentType))
                        request.ContentType = contentType;
                    if (!string.IsNullOrEmpty(userAgent))
                        request.UserAgent = userAgent;
                    if (cookieContainer != null)
                        request.CookieContainer = cookieContainer;

                    request.Timeout = Constants.RequestTimeOut;

                    if (request.Method == "POST")
                    {
                        if (postData != null)
                        {
                            request.ContentLength = postData.Length;
                            using (var dataStream = request.GetRequestStream())
                            {
                                dataStream.Write(postData, 0, postData.Length);
                            }
                        }
                    }

                    using (var httpWebResponse = request.GetResponse() as HttpWebResponse)
                    {
                        if (httpWebResponse != null)
                        {
                            responseUri = httpWebResponse.ResponseUri.AbsoluteUri;
                            cookieContainer.Add(httpWebResponse.Cookies);
                            using (var streamReader = new StreamReader(httpWebResponse.GetResponseStream()))
                            {
                                result = streamReader.ReadToEnd();
                            }
                            return result;
                        }
                    }
                }
            }
            responseUri = null;
            return null;
        }

For doing FORM posts, the best way is to use WebClient.UploadValues() with a POST method.

Hope this works

webRequest.Credentials= new NetworkCredential("API_User","API_Password");

I have a feeling that the username and password that you are sending should be part of the Authorization Header. So the code below shows you how to create the Base64 string of the username and password. I also included an example of sending the POST data. In my case it was a phone_number parameter.

string credentials = Convert.ToBase64String(Encoding.ASCII.GetBytes(_username + ":" + _password));

HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(Request);
webRequest.Headers.Add("Authorization", string.Format("Basic {0}", credentials));
webRequest.ContentType = "application/x-www-form-urlencoded";
webRequest.Method = WebRequestMethods.Http.Post;
webRequest.AllowAutoRedirect = true;
webRequest.Proxy = null;

string data = "phone_number=19735559042"; 
byte[] dataStream = Encoding.UTF8.GetBytes(data);

request.ContentLength = dataStream.Length;
Stream newStream = webRequest.GetRequestStream();
newStream.Write(dataStream, 0, dataStream.Length);
newStream.Close();

HttpWebResponse response = (HttpWebResponse)webRequest.GetResponse();
Stream stream = response.GetResponseStream();
StreamReader streamreader = new StreamReader(stream);
string s = streamreader.ReadToEnd();