Registering XML callbacks

I have been trying to set up an API that posts XML to a server and then
recieves an XML response back. I took the showxml example and the postform
example that come with the library and tried to patch them together, but no
matter what I try, the xml callbacks are never called. The key area of my
code is as follows

HTParseFormInput(form, XML);

HTRequest_setOutputFormat(request, WWW_PRESENT);
//HTRequest_setOutputFormat(request, HTAtom_for("text/xml"));

anchor = HTAnchor_findAddress(uri);

HTPostFormAnchor(form, anchor, request);

depending on the outputformat, results vary from storing response in a
temporary file to displaying on stdout to just disappearing.

Any help would be greatly appreciated.

John Fisher