Set default dropdown values in Zendesk Help Center. This is just the code to set the actual value.

hc_ticket_fields.html

<div class="form-field string required request_custom_fields_21631456">
  <label for="request_custom_fields_21631456">fav</label>
  <input autocomplete="off" data-tagger=data-tagger="{"-":{"id":"","title":"-","selected":true},"pet":{"cat":{"id":"cat","title":"pet::cat"},"dog":{"id":"dog","title":"pet::dog"}},"Dolphin":{"id":"dolphin","title":"Dolphin"},"the fish":{"id":"the_fish","title":"the fish"}}" id="request_custom_fields_21631456" name="request[custom_fields][21631456]" size="0" type="hidden" value="">
  <a class="nesty-input" tabindex="0">-</a>
</div>
<script>
//in order to set the value of the field and make it visible set the input value and anchor text 
//so this will set the field fav to The fish
     $('.request_custom_fields_21631456').one('DOMNodeInserted', function(d){
     console.log(d);
     $(d.currentTarget).val('the_fish');
     $(d.target).text('The Fish');
   });
</script>