JSFiddle - React, Tailwind, and code Playground

HTML

<select id="my_SiteUsers" style="width:200px;">
    <option value='default'>Select a user</option>
    <option value='2'>Select a user 2</option>
</select>

JavaScript

window.userValue = '';

$(document).ready(function(){
    $('#my_SiteUsers').on('change', function(){
        RefreshGroupLists();
    });
});

function RefreshGroupLists(){
  userValue = $('#my_SiteUsers').find(':selected').val();
  alert(userValue);
}