文本框显示Ajax的值

Question

使用PHP，Ajax和Jquery，我能够创建一个级联相关的下拉菜单。但我希望将第三个下拉列为文本框。

我该怎么做？

Ajax Code PHP代码......

<?php
    include('conn.php');
     $query = $con->query("SELECT * FROM job_category group by category_name ");
    $rowCount = $query->num_rows;
    ?>

    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;  
    Category:
    <select name="category" id="category">
        <option value="">Select Category</option>
        <?php
        if($rowCount > 0){
            while($row = $query->fetch_assoc()){ 
                echo '<option value="'.$row['category_num'].'">'.$row['category_name'].'</option>';
            }
        }else{
            echo '<option value="">Country not available</option>';
        }
        ?>
    </select>

    <br>
    &nbsp;&nbsp;Sub-Category:
    <select name="sub_type" id="sub_type">
    </select>
    <br>

  &nbsp;&nbsp;
        &nbsp;&nbsp;&nbsp;Priority:
   <select name="priority" id="priority_level" >

  </select>
 ?>

请帮助，我在这方面遇到了困难。