SOURCE

<script>
function start() {
  /* Start application when in correct orientation */
  rotate()
}
async function rotate() {
  try {
    await screen.orientation.lock("landscape");
    start();
  } catch (err) {
      alert(err)
    // console.error(err);
  }
  const matchLandscape = matchMedia("(orientation: landscape)");
  if (matchLandscape.matches) return start();
  addEventListener("orientationchange", function listener() {
    matchLandscape.addListener(function mediaChange(e) {
      if (!e.matches) return;
      removeEventListener("orientationchange", listener);
      matchLandscape.removeListener(mediaChange);
      start();
    });
  });
  alert("To start, please rotate your screen to landscape.");
}
</script>
<button onclick="start()">
  lock
</button>