public class MessageBase<T>
{
    public MessageBase() { this.message = new List<T>(); }
    public string type { get; set; }
    public string id { get; set; }
    public List<T> message { get; set; }
}

public class LogonRequest
{
    public string userName { get; set; }
    public string password { get; set; }
}

MessageBase<LogonRequest> logon = new MessageBase<LogonRequest>()
{
    id = Guid.NewGuid().ToString(),
    type = "Logon"
};

logon.message.Add(new LogonRequest() {userName = "user", password = "123"});

var message = (new JavaScriptSerializer()).Serialize(logon);

var messageObj = {id: 123456789,
                  type: 'Logon',
                  message:[{username:'user', password:'123' }]
                 };

var message = JSON.stringify(messageObj);