MyWebService.MyWebServiceClient client = new MyWebService.MyWebServiceClient();

client.ClientCredentials.Windows.AllowedImpersonationLevel = System.Security.Principal.TokenImpersonationLevel.Impersonation;
client.ClientCredentials.Windows.ClientCredential = new NetworkCredential(username, password, domain);

<security mode="TransportCredentialOnly">
    transport clientCredentialType="Windows" realm="" />
<security>