POST to REST API from winform returns empty

I have a project that uses the hockeystreams.com API. I am having trouble with using their API to login from my WinForm. It always gives me a blank response, whether I give a bad username/password combination or a correct one. My code is as follows:

public static bool LogIn(string username, string password)
    {
        string URL = string.Format("{0}&username={1}&password={2}", Global.LOGIN_API, username, password);
        HttpWebRequest webRequest = (HttpWebRequest)HttpWebRequest.Create(URL);
        webRequest.ContentType = "application/json; charset=utf-8";
        webRequest.Accept = "application/json, text/javascript, */*";
        webRequest.Method = "POST";
        try
        {

            WebResponse response = webRequest.GetResponse();
            Stream stream = response.GetResponseStream();
            string json = "";

            using (StreamReader reader = new StreamReader(stream))
            {
                while (!reader.EndOfStream)
                {
                    json += reader.ReadLine();
                }
            }

            LogInJSON deserializedLogInData = JsonConvert.DeserializeObject<LogInJSON>(json);
            if (deserializedLogInData.Status == "Success")
            {
                Global.TOKEN = deserializedLogInData.Token;
                Global.USERNAME = deserializedLogInData.Username;
                Global.ISPREMIUM = deserializedLogInData.Membership == "REGULAR" ? false : true;
                Global.FAVORITETEAM = deserializedLogInData.Favteam;

                return true;
            }
            else
            {
                return false;
            }
        }

I have tried changing the POST to PUT, but that throws a 406 error (Bad Request). I am very green with JSON so I don't know where to go from here. Any help greatly appreciated!