var restClient = new JsonServiceClient(...);
var authenticate = restClient.Post<AuthService>(
"/auth/twitter?format=json",
new Auth()
{
    UserName = "username",
    Password = "password",
    RememberMe = true
 });

var restClient = new JsonServiceClient(...);
var authenticate = restClient.Post(new Auth {
    provider = "twitter",
    UserName = "username",
    Password = "password",
    RememberMe = true
});