I assume that just pressed is async with respect to html events which then will not work. Perhaps you can call addEventListener on window and catch Enter or F11. That should count as user initiated as well.
I am new to haxe so here is the js code that works.
window.addEventListener("keydown", function(ev) {
if(ev.key=="F11" /* or Enter if you want */) {
var thing = document.getElementById("mything");
thing.webkitRequestFullscreen();
ev.preventDefault();
ev.stopPropagation();
}
});
