Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<p>Click the button to create an OPTGROUP element.</p>

<button onclick="myFunction()">Try it</button>

<select id="mySelect" size="6">
  <option>BMW
  <option>Mercedes
  <option>Ford
  <option>Volvo
  <option>Toyota
</select>

<script>
function myFunction() {
    var x = document.getElementById("mySelect");
    var y = document.createElement("OPTGROUP");
    y.setAttribute("label", "SwedishCars");
    y.appendChild(x.options[3])
    x.insertBefore(y, x.options[3]);
}
</script>

</body>
</html>