Advice, how to create Post header in squid

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http://nixcraft.com – hi, i have a little trouble using squid headers. i explain. i want that when a client do request from spesific Url, like: test.com/login.php the squid automaticliy send post header like: test.com/login.asp 'POST' username=bob&password=1234567. the goal is when client using the proxy server he will be automaticliy logged to the server. i dont know where to start. do i need to create my own header? if not how can i possibly send post data. (HowTos)