var win = window.open('https://www.youtube.com/embed/VLSA9oVneLA?autoplay=1', '_blank');
but I don’t know how to make it automatically go fullscreen from Playcanvas. There doesn’t seem to be anything I can add to the URL in the YouTube API.
The iframe is adding a DOM element to the page. Some examples of this (https://developer.playcanvas.com/en/tutorials/htmlcss-ui/) is this but you will have to make it work with an iframe instead. w3schools should help with the CSS and HTML side of it