C# login API for one site

I need you to fix login API (function, set of functions). This functions must be written into C#. I have application with all functions and classes, that you need to solve this problem. This classes and functions works good for all other sites (PHP, Perl,Python, etc.). But this site has written into ASP.NET (I have not access to source code of this site or any other clues). So this functions not work with this site. Also I can't understand why, but this site not allow to login with success Fiddler webssession (I used Fiddler->Composer and entered valid session RAW data, but got not logged user page). So you need to understand what params you need to pass into POST request to got successfully logged user (text "rua-l-auth rua-l-auth_logged" must be into result page). I wrote all gear for you. You just need open Fiddler (or other preferred by you HTTP debugging) and understand what is wrong and fix my code. Sample application use SgmlReader library. You can find it into NuGetPackages or into internet. I will give you my login and password to site. Site name you can find into sources.