JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://code.jquery.com/jquery-3.1.1.min.js"></script>
<select field_type="multiselect" name="geodir_property_features[]" id="geodir_property_features" multiple="multiple" class="geodir_textfield textfield_x chosen_select" data-placeholder="Select" option-ajaxchosen="false" style="/* display: none; */">
                    <option value="">Select Features</option><option value="Gas Central Heating">Gas Central Heating</option><option value="Oil Central Heating">Oil Central Heating</option><option value="Double Glazing">Double Glazing</option><option value="Triple Glazing">Triple Glazing</option><option value="Front Garden">Front Garden</option><option value="Garage">Garage</option><option value="Private driveway">Private driveway</option><option value="Off Road Parking">Off Road Parking</option><option value="Fireplace">Fireplace</option></select>
                    
                    <input type='text' id='post_tags' value='' />

JavaScript

jQuery('#geodir_property_features').change(function() {
  
  jQuery('#post_tags').val(jQuery('#geodir_property_features').val().join(","));
  
});