<script>
jQuery(document).ready(function($) {
  $('.book_now_select select').on('invalid', function(e) {
    e.preventDefault();
    this.setCustomValidity('Please select from the list');
    $(this).addClass('error');
  });

  $('.book_now_select select').on('change', function(e) {
    e.preventDefault();
    if ($(this).val() == '') {
      this.setCustomValidity('Please select an he list');
      $(this).addClass('error');
    } else {
      this.setCustomValidity('');
      $(this).removeClass('error');
    }
  });
});
</script>