Integration

I'm trying to invoke a Siebel CRM OnDemand Web Service via BPEL. Basically, the Siebel guide says to post to a url with user/pwd to get a session ID returned from a cookie. I did that using embedded java and have the session ID as a variable in my BPEL process. My problem is with now calling the Web Service to perform the action. I have a WSDL from Siebel, but the guide says to use an HTTP post with the session ID in the url and the XML in the body. How do I do that? Shouldn't the WSDL handle that when I create the Partner Link? Do I need to use more embedded Java for this or can I set up a Partner Link? I created a Partner Link and it shows the methods I want to use, but I get the following error when I try to call it because I don't know where to add the session ID:
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<ErrorCode xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">SBL-ODU-01006</ErrorCode>
<ErrorMessage xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">Internal Error: Session is not available. Aborting.</ErrorMessage>
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Any help would be GREATLY appreciated. Please let me know if you need more information.
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THANKS!