JSFiddle - React, Tailwind, and code Playground

HTML

<select id="assignedLinguistListBox">
  <option>foo</option>
  <option>bar</option>
  <option>baz</option>
</select>
<input type="hidden" id="assignedIDHiddenInput" />

JavaScript

var s = $('#assignedLinguistListBox option').map(function() { 
          return this.value; 
        }).get().join(',');
$('#assignedIDHiddenInput').val(s);

alert($('#assignedIDHiddenInput').val());