<form action="" method="POST">
<select name="sity" class="sity">
<option data-id="11" value="1">Минск</option>
<option value="2">Брест</option>
<option value="3">Пинск</option>
</select>
<input type="submit" name="go">
</form>
<script
src="
https://code.jquery.com/jquery-3.4.1.js"
integrity="sha256-WpOohJOqMqqyKL9FccASB9O0KwACQJpFTUBLTYOVvVU="
crossorigin="anonymous"></script>
<script type="text/javascript">
$('.sity').on('change', function() {
alert($(this).data('id'));
});
</script>