Makes it clearer, thanks.
NG client runs in the browser and the actual Servoy client runs on the server. So this
file:///C:/Users/walter/Documents/tests/playground.html
will not be possible, because you can't access the file system like that from a browser.
Without knowing a bit more about what your game looks like, it's hard to say what the best solution is. Running in the browser means you are inside an html page already and you can't have two, unless you use an iframe. Probably, the "NG way" of doing this would be to wrap your game in a web component (see
https://wiki.servoy.com/display/DOCS/WebComponents) that you can place where you like, call code in it and handle callbacks. Simply "injecting" your page could be more tricky, but that highly depends on what your thing looks like. In the smart client this appears more straight forward, but that is because every jfxpanel is its own browser (html page). With NG (or Web) client you are already in an html page...