Как заполнить формы и отправить с Webclient в C#

Вероятно, вам следует использовать HttpWebRequest для этого. Вот простой пример:

var strId = UserId_TextBox.Text;
var strName = Name_TextBox.Text;

var encoding=new ASCIIEncoding();
var postData="userid="+strId;
postData += ("&username="+strName);
byte[]  data = encoding.GetBytes(postData);

var myRequest =
  (HttpWebRequest)WebRequest.Create("http://localhost/MyIdentity/Default.aspx");
myRequest.Method = "POST";
myRequest.ContentType="application/x-www-form-urlencoded";
myRequest.ContentLength = data.Length;
var newStream=myRequest.GetRequestStream();
newStream.Write(data,0,data.Length);
newStream.Close();

var response = myRequest.GetResponse();
var responseStream = response.GetResponseStream();
var responseReader = new StreamReader(responseStream);
var result = responseReader.ReadToEnd();

responseReader.Close();
response.Close();

Вы уже отправили его с UploadValues ​​. Вопрос в том, каков ваш «поиск результатов»? Что возвращает страница? HTML? Если это так, то HTML Agility Pack - самый простой способ разбора html.

I have found a solution to my problem. First of all I was confused about some of the basics in http-communication. This was caused by a python-script i wrote, which have a different approach with the communication.

I solved it with generating the POST-data from scratch and open the uri, which was contained in the form-action.