Implementing Last.fm authentication in Windows Phone 8

Question

I need to implement last.fm authentication in my Windows Phone 8 application. Last.fm requires a POST request with https for successful login. Here is what I'm doing.

public async void GetMobileSession(string userName, string password, Action<LastFmAuthResponse> onCompletion)
    {
        string CORE_URL = "https://ws.audioscrobbler.com/2.0/";
        var parameters = new Dictionary<string, string>();
        parameters.Add("username", userName);
        parameters.Add("password", password);
        parameters.Add("method", "auth.getMobileSession");
        parameters.Add("api_key", api_key);

        string signature = GetSignature(parameters);

        string comboUrl = string.Concat("method=auth.getMobileSession", "&api_key=", api_key,
            "&username=", userName, "&password=", password, "&api_sig=", signature);

        LastFmAuthResponse response = null;

        byte[] pendingPostContent = Encoding.UTF8.GetBytes(comboUrl);

        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(CORE_URL);
        request.Method = "POST";
        request.ContentType = "application/x-www-form-urlencoded";

        using (Stream requestStream = await request.GetRequestStreamAsync())
        {
            await requestStream.WriteAsync(pendingPostContent, 0, pendingPostContent.Length);
        }

        request.BeginGetResponse(new AsyncCallback(n =>
        {
            HttpWebResponse rawResponse = (HttpWebResponse)request.EndGetResponse(n);

            string rawData = string.Empty;
            using (StreamReader reader = new StreamReader(rawResponse.GetResponseStream()))
            {
                rawData = reader.ReadToEnd();
            }

            try
            {
                if (!string.IsNullOrEmpty(rawData))
                {
                    response = CommonFunctions.GetObjectFromString<LastFmAuthResponse>(rawData);
                }
            }
            catch
            {
            }

            onCompletion(response);

        }), null);
    }

However the code is failing at request.BeginGetResponse It returns an error saying remote server could not be found. Can someone please point out what I'm doing wrong here?