Midel,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
</head>
<body>
<select id="test" name="" size="1" onchange="alert(this.value)">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
<script>
document.getElementById('test').value = 2 ;
document.getElementById('test').onchange()
</script>
</body>
</html>