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; }