I use C++ code (QNetworkAccessManager etc) to get the url. I've attached a text file containing the methods. I basically use the get() method of QNetworkAccessManager and connect the finished() signal to a slot where the response html is parsed. You pass the YouTube link to the getVideoUrl() method. My YouTube class has a playbackFormat attribute which holds the stream format (5 or 18). This is used in the parseVideoPage() slot, but you could just hard-code a value if you prefer.

Note that it was necessary to create a new QNetworkAccessManager object each time the method is called, otherwise I would get a 403 response when attempting to play the stream (after the first video).