I\'m using webbrowser control to login any site. And then i want to download some sub page html using WebRequest (or WebClient). This links must requires authentication.
I've actually been through this same problem on the Windows Mobile platform, and the only thing that worked was to extend the WebBrowser control (using C++ :<) to capture the POST/GET vars before the request is sent.
This library may help you out:
http://www.codeproject.com/KB/miscctrl/csEXWB.aspx
"..the library implements the PassthroughAPP package by Igor Tandetnik, which enables the client to intercept all HTTP and HTTPS request and responses."
So although it is not possible to get the POST/GET vars used for your basic auth on a standard WebBrowser control, it would be possible if you use an extended control such as the sample I linked - in fact many "Extended WebBrowser" controls are created because of problems very similar to yours. Unfortunately, as far as I know you need to do this using unmanaged code/c++ :(.