<!-- required files -->
<link href="../assets/plugins/select-picker/dist/picker.min.css" rel="stylesheet" />
<script src=".。/assets/plugins/select-picker/dist/picker.min.js"></script>

<!-- html -->
<select class="selectpicker form-control" id="ex-search" multiple>
  ...
</select>

<!-- script -->
<script>
  $('#ex-search').picker({ search: true });
</script>