<html>
<body>
<select name="users" id="users">
<option value="User 1">User 1</option>
<option value="User 2">User 2</option>
<option value="User 3">User 3</option>
<option value="User 4">User 4</option>
</select>
<script
src="https://code.jquery.com/jquery-3.4.0.slim.min.js"
integrity="sha256-ZaXnYkHGqIhqTbJ6MB4l9Frs/r7U4jlx7ir8PJYBqbI="
crossorigin="anonymous"></script>
<script>
$(document).ready(function() {
$('#users').change(function() {
document.location.href = "/search.php?query=" + $('#users option:selected').val();
});
});
</script>
</body>
</html>