Asp.net 如何获得EVENTVALIDATION？

我正在尝试模拟登录Asp.NETWebForms页面。我使用fiddler来比较“模拟”登录和真实登录。请求中唯一缺少的是uu EVENTVALIDATION的值，但我不知道如何捕获EVENTVALIDATION变量，以便在请求中发送它。 我的代码如下所示：

TextWriter tw = new StreamWriter("test.txt");
CookieContainer cookieJar = new CookieContainer();
HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create("http://myserver.com/Start.aspx?g=36");
request.CookieContainer = cookieJar;
HttpWebResponse response = (HttpWebResponse)request.GetResponse();

CookieContainer cookies = new CookieContainer();
HttpWebRequest request1 = (HttpWebRequest)WebRequest.Create("http://myserver.com/login/Login.aspx");
request1.CookieContainer = cookieJar;
HttpWebResponse response1 = (HttpWebResponse)request1.GetResponse();
//response1.Cookies = cookieJar.GetCookies(request.RequestUri);
StreamReader reader = new StreamReader(response1.GetResponseStream());
string loginPage = reader.ReadToEnd();
reader.Close();

string viewState = GetViewState(loginPage);
string postback = String.Format("__VIEWSTATE={0}&ctl00$ContentPlaceHolder1$ctl01$UserName={1}&ctl00$ContentPlaceHolder1$ctl01$Password={2}&ctl00$ContentPlaceHolder1$ctl01$LoginButton=Logga in", viewState, "myusername", "mypassword");
HttpWebRequest request2 = (HttpWebRequest)WebRequest.Create("http://myserver.com/login/Login.aspx");
request2.Method = "POST";
request2.ContentType = "application/x-www-form-urlencoded";
request2.CookieContainer = cookieJar;
StreamWriter writer = new StreamWriter(request2.GetRequestStream());
writer.Write(postback);
writer.Close();
request2.GetResponse().Close();

我需要在最后一个请求中附加eventvalidation字段，但我不知道必须捕获eventvalidation的值