如何从下拉框获取数据并在js中添加下一个框

Question

</div> 

<!--/form-group-->             
<label for="clientContact" class="col-sm-3 control-label">Select % For Gst </label>
<div class="col-sm-9">
  <select class="form-control" name="gst1" id="gst1" onkeyup="getgstvalueFunc()" autocomplete="off" >
    <option value="">~~SELECT~~</option>
    <option value="1">5</option>
    <option value="2">12</option>
    <option value="3">18</option>
    <option value="4">28</option>
  </select>
  <input type="hidden" class="form-control" id="gst1Value" name="gst1Value"  />
</div>
</div> <!--/form-group-->
<div class="form-group">
<label for="gst" class="col-sm-3 control-label">Gst in %</label>
<div class="col-sm-9">
  <input type="text" class="form-control" id="gst" name="gst" disabled="true" value="<?php echo $data[5] ?> "/>
  <input type="hidden" class="form-control" id="gstValue" name="gstValue" value="<?php echo $data[5] ?>" />
</div>
</div> <!--/form-group-->




// gst
var gst = (Number($("#subTotal").val())/100) * Number($("#gst1").val());
gst = gst.toFixed(2);
$("#gst").val(gst);
$("#gstValue").val(gst);