Righty. I've been given an API specification for a particular web service. I know that I have to send their service some XML, and it'll send some back. And then theirs will connect back to mine and send some XML.

Assuming that I will be given a URL to their service, and I give them a URL to mine - how do I actually send an arbitrary section of XML via HTTP POST to something?
If I create a pretend form and send the data by post it'll just send it as normal html crap post - so how do I actually send XML?