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<!DOCTYPE html>
<html>
<body>

<form>
  <select id="mySelect" size="8">
    <option>Apple</option>
    <option>Pear</option>
    <option>Banana</option>
    <option>Orange</option>
  </select>
</form>
<br>

<p>Select an option in the dropdown list. Then click the button below to add a "Kiwi" option before the selected option.</p>

<button type="button" onclick="myFunction()">Insert option before selected</button>

<script>
function myFunction() {
    var x = document.getElementById("mySelect");
    if (x.selectedIndex >= 0) {
        var option = document.createElement("option");
        option.text = "Kiwi";
        var sel = x.options[x.selectedIndex];  
        x.add(option, sel);
    }
}
</script>

</body>
</html>