<html>
<head>
<script>
function displayResult()
{
document.getElementById("mySelect").multiple=true;
}
</script>
</head>
<body>
<form>
<select id="mySelect" size="4">
<option>Apple</option>
<option>Pear</option>
<option>Banana</option>
<option>Orange</option>
</select>
</form>
<button type="button" onclick="displayResult()">Allow multiple selection</button>
<p>Hold Ctrl to select more than one option.</p>
</body>
</html>