IServiceManagement<IDiscoveryService> serviceManagement =
                        ServiceConfigurationFactory.CreateManagement<IDiscoveryService>(discoveryUri);
ClientCredentials credentials = new ClientCredentials();
credentials.Windows.ClientCredential = new System.Net.NetworkCredential(userName, password, domain);
using (DiscoveryServiceProxy serviceProxy = new DiscoveryServiceProxy(new DiscoveryServiceProxy(serviceManagement, credentials))
{
    RetrieveOrganizationsRequest orgRequest = new RetrieveOrganizationsRequest();
    RetrieveOrganizationsResponse orgResponse =
                (RetrieveOrganizationsResponse)service.Execute(orgRequest);
    // do something with organisations
}