Having trouble sending JSON & Key via POST

Qt Code: 
  1. QByteArray jsonString = "{}";
  2.  
  3.     QByteArray postDataSize = QByteArray::number(jsonString.size());
  4.  
  5.     QUrl req("webpagehere");
  6.     req.setQuery("Key=XXXXXXXXXXXXX");
  7.  
  8.     QNetworkRequest request(req);
  9.  
  10.     request.setRawHeader("User-Agent", "Test");
  11.     request.setRawHeader("X-Custom-User-Agent", "Test");
  12.     request.setRawHeader("Content-Type", "application/json");
  13.     request.setRawHeader("Content-Length", postDataSize);
  14.  
  15.     QNetworkAccessManager test;
  16.  
  17.     QEventLoop loop;
  18.     connect(&test, SIGNAL(finished(QNetworkReply*)), &loop, SLOT(quit()));
  19.     QNetworkReply * reply = test.post(request, jsonString);
  20.     connect(reply, SIGNAL(error(QNetworkReply::NetworkError)), this, SLOT(onError(QNetworkReply::NetworkError)));
  21.     loop.exec();
  22.  
  23.     QByteArray response = reply->readAll();
  24.     qDebug() << response;
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Is there anything wrong with this code? or is it my webpage which is at fault? or possibly both?

Regards,

Richard