Please try the following code: I'm using cdn for jquery multiselect in the below example. You can find the latest cdn here: https://cdnjs.com/libraries/multi-select
<html>
<head>
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/multi-select/0.9.12/css/multi-select.css" integrity="sha512-2sFkW9HTkUJVIu0jTS8AUEsTk8gFAFrPmtAxyzIhbeXHRH8NXhBFnLAMLQpuhHF/dL5+sYoNHWYYX2Hlk+BVHQ==" crossorigin="anonymous" referrerpolicy="no-referrer" />
</head>
<body>
<select multiple="multiple" id="my-select" name="my-select[]">
<option value='elem_1'>elem 1</option>
<option value='elem_2'>elem 2</option>
<option value='elem_3'>elem 3</option>
<option value='elem_4'>elem 4</option>
<option value='elem_100'>elem 100</option>
</select>
<script src="https://code.jquery.com/jquery-3.6.0.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/multi-select/0.9.12/js/jquery.multi-select.js" integrity="sha512-X1iMoI6a2IoZFOheUVf3ZmcD1L7zN/eVtig6enIq8yBlwDcbPVao/LG8+/SdjcVn72zF+A/viRLPSxfXLu/rbQ==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>
<script type="text/javascript">
$('#my-select').multiSelect();
</script>
</body>
</html>