Hey,

You simply need to craft your query properly.

1) Is the form method POST or GET? This will change how the query is crafted. Your example makes use of a GET (no body, simply URL parameters).

2) Is the action a URL or javascript? Is there an action? Does the login button have javascript for onClick that overrides the action?

Then you simply need to format your query properly... Since POST would be the proper approach for a login, let's assume that's used. Using "Nickname" and "Password" as the actual values, and assuming the action page is "/default.aspx", you'd compose a request that looks something like this
Code:
POST /default.aspx HTTP/1.1
HOST: example.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 105

__VIEWSTATE=dDwzNTU2MjMyMTU7dDw7b&MemberSignin1%3AtbNickname=Nickname&MemberSignin1%3AtbPassword=Password
I'm assuming my count wasn't off on Content-Length... it's a count of the body characters... Note that the : was replaced with %3A.