I figured it out:
QStringList propertyNames;
QStringList propertyKeys;
QString strReply = (QString)reply->readAll();
QJsonDocument jsonResponse = QJsonDocument::fromJson(strReply.toUtf8());
QJsonObject jsonObject = jsonResponse.object();
QJsonArray jsonArray = jsonObject["properties"].toArray();
foreach (const QJsonValue & value, jsonArray) {
QJsonObject obj = value.toObject();
propertyNames.append(obj["PropertyName"].toString());
propertyKeys.append(obj["key"].toString());
}