I have this:

1.    function getRequestDescriptions(requestCategory) {
      $.ajax({
       url: 'getRequestDescri.asp',
       dataType: 'json',
       data: { requestCategory: requestCategory }
      })
      .done(function(descriptionInfo) {
    
            // get number of items in array given by asp
            var Desc_count = descriptionInfo.length;
    
            // start an array to hold re-arranged buildings array
            var newDescriptionInfo = [];
    
            // loop request descriptions
            for (var i = 0; i < Desc_count; i += 1) {
    
                // append an <option> tag to your <select>
                $('#description').append('<option value="' + descriptionInfo[i].RequestDescriptionDisplay + '">' + descriptionInfo[i].RequestDescriptionDisplay + '</option>');
    
                // set the key value to SiteContactDisplay so that it is easily referenced later
                newDescriptionInfo['' + descriptionInfo[i].RequestDescriptionDisplay + ''] = descriptionInfo[i];
    
            }
    
    
            // Listen for the value of the <select> to change
            $('#description').on('change', function () {
    
                // Hidden inputs must already be available on the form before this Javascript is ever used
                // Set the value of the hidden fields based on the <select>'s ID choosing the corret array element
                $('input[name="RequestID"]').val(newDescriptionInfo[''+$(this).val()+''].RequestID);
            });
        });
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   
   

Then this:

1.   <input name="RequestID" id="RequestID" type="text"  width="300" value=""  class="changeable" /><br>
     <select name="description" id="description" style="width:600px;font-size:10pt;"  class="changeable" data-summary="summSubCategory">
   

A user selects an option from Category dropdown (not shown) then values associated with that Category are populated into the description dropdown.