Login from a site to another site in https

i'm new to stackoverflow and I have a question for you:

I have to do an automatic login from my wesite to another external website (not mine).

This site use https and when i go on the site address it ask me (with the classical popup form) to enter username and password.
I've tried to do a POST in this way

http://www.terminally-incoherent.com/blog/2008/05/05/send-a-https-post-request-with-c/

where in the string post_data I have put "Username=xxxx&Password=xxxx"

but doesn't work; the code returned was 401: Unauthorized.

Any suggestions?

Thank you!

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