Windows 8 store app Dropbox get access token and upload issue

I have used the following url, app key, app secret and data which has posted to retrieve the access token.

URL: https://api.dropbox.com/1/oauth/access_token?oauth_token=KDIGaDoXjQBvT0pn

AppKey = "qas1ok71ay8wqr0"
AppSecret = "<REDACTED>"

Posted Data : OAuth oauth_consumer_key="qas1ok71ay8wqr0", oauth_nonce="300571600", oauth_signature_method="HMAC-SHA1", oauth_timestamp="1428469092", oauth_version="1.0", oauth_signature="tJTlNcaoYq6J01FwOjflR8s37P8%3D"

GetAccess Token Method Code:

public async void GetAccessToken(string url, string key, string secret)
{
    TimeSpan SinceEpoch = (DateTime.Now - new DateTime(1970, 1, 1, 0, 0, 0, 0).ToLocalTime());
    Random Rand = new Random();

    Int32 Nonce = Rand.Next(1000000000);

    String sigBaseStringParams = "oauth_consumer_key=" + _apiKey;        
    sigBaseStringParams += "&" + "oauth_nonce=" + Nonce.ToString();
    sigBaseStringParams += "&" + "oauth_signature_method=HMAC-SHA1";
    sigBaseStringParams += "&" + "oauth_timestamp=" + Math.Round(SinceEpoch.TotalSeconds);       
    sigBaseStringParams += "&" + "oauth_version=1.0";
    String SigBaseString = "POST&";
    SigBaseString += Uri.EscapeDataString(url) + "&" + Uri.EscapeDataString(sigBaseStringParams);
    var signature = GenerateSignature(SigBaseString, secret);

    String DataToPost = "OAuth oauth_consumer_key=\" + _apiKey + "\", oauth_nonce=\" + Nonce.ToString() + "\", oauth_signature_method=\"HMAC-SHA1\", oauth_timestamp=\" + Math.Round(SinceEpoch.TotalSeconds) + "\", oauth_version=\"1.0\", oauth_signature=\" + Uri.EscapeDataString(signature) + "\";

    var postResponse = await PostData(url, DataToPost);
}
private async Task<string> PostData(String Url, String Data)
{

    HttpWebRequest Request = (HttpWebRequest)WebRequest.Create(Url);
    Request.Method = "POST";
    Request.Headers["Authorization"] = Data;
    HttpWebResponse Response = (HttpWebResponse)await Request.GetResponseAsync();
    StreamReader ResponseDataStream = new StreamReader(Response.GetResponseStream());
    return ResponseDataStream.ReadToEnd();

}