Web Proxy Server: Posting data on https websites

Question

I am developing a web proxy server on VS C# .NET, currently as a console application. I am using HttpWebRequest and HttpWebResponse for communicating with any server. When the client enters login name and password on https websites through this proxy server, the proxy takes this request and then tries to post this data to https websites. But my proxy is unable to. Yahoo mail says that 'The browser refuses to sign in (cookies rejected)'. Please tell me what do https websites expect from a browser, and also how to make my proxy communicate properly with https websites. Waiting for a reply, Thanks in anticipation