Adding an action to a POST Using HttpWebRequest

I am writing a service to retrieve data via a HttpWebRequest with the method
set to POST. I am having some trouble getting a correct response. I need to
be able to set the action on the POST request apart from the URL. The
documentation on the Web server says the POST should look something like:

POST /app/AnAppServlet HTTP/1.1

However when I append this to the address in the URL, the app locks up. The
Web server us running HTTPS with a client certificate. My code seems to
handle the certificate OK if I set the URL to simply address the Web server,
except it responds with an empty Web page. I need to invoke a servlet on the
server.

I see a way to set a SOAPAction via a request header but this is not SOAP.

Any help would be appreciated.

Thanks,
Eagle