Hi,

First of all, a message can not be easily signed when you are posting the
message directly to a http endpoint.
You will need to change your code to use a proxy class instead, .NET
(wsdl.exe) and WSE (wsdlwse3.exe) provide tools to auto generate that class
using the web service's WSDL. Once you have the proxy class, you need to
configure the proxy to use a WSE security policy (The policy says that the
message has to be signed by WSE).
When your client application invokes the service through the auto-generated
proxy, it will automatically be signed by WSE. Configuring WSE can be easily
done through the WSE configuration tool, it provides a set of wizards to
specify the security requeriments for the service.

Regards,
Pablo Cibraro.