您的表单数据不正确。删除＆放后，在开始它为我工作：

Your form data is incorrect. After removing the & at the beginning it worked for me:

sw.Write(string.Format("loginname={0}&password={1}&btnSubmit=Log In&institutioncode=H4V9KLUT45AV&version=2", Username, PassWord));

此外，如在其他答案已经提到的，您需要添加返回 ASPSESSIONIDs-s-rRDRST cookie的进一步请求到该网站。

Additionally, as already mentioned in the other answer, you need to add the returned ASPSESSIONIDs-s-rRDRST cookie in further requests to the site.