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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>Click the button to add a "Kiwi" option at the end of the dropdown list.</p>

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

<script>
function myFunction() {
    var x = document.getElementById("mySelect");
    var option = document.createElement("option");
    option.text = "Kiwi";
    x.add(option);
}
</script>

</body>
</html>