XML digital signature interface

Question

I am trying to call a WebService and it requires me to prepare an XML digital signature interface. I came across this website and it shows how to sign an XML document. My problem now is that I do not create the XML document myself. My code is as follows:

string myResult;

GenericWS.ServicesService a = new GenericWS.ServicesService();
GenericWS.Service2 b = new GenericWS.Service2();

b.Something = "3";

X509Certificate cert = X509Certificate.CreateFromCertFile("C:\\somepath\\somecert.cer");

a.ClientCertificates.Add(cert);

ServicePointManager.ServerCertificateValidationCallback = delegate
                (object s, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
            { return true; };

myResult = a.WSRequest(b).WSReturnValue.ToString();

Label1.Text = myResult;

Basically, the WSRequest() will generate the body of the SOAP message. However, in order to sign the SOAP message, I need to pass in the XML file that needs to be signed.

I tried to serialize the object and cast it as XML and pass it into the signature interface but I do not have another method that I can invoke to call the webmethod.

Is there any other solution?