我如何放置一個添加按鈕，用戶可以在 html 的選擇列表中添加他的選擇

Question

<label  for="Course"> Course </label>
<select name="course" >
<option>select your course`</option> 
<?php

$query="SELECT Course_Name FROM courses";
$result=mysqli_query($conn,$query);
if($result)
{
    while($row=mysqli_fetch_array($result)) {
        $Course_Name=$row["Course_Name"];

        echo "<option value='".$Course_Name."'>".$Course_Name." </option>";

    }
}

?>
</select>

Answer 1

<label  for="Course"> Course </label>
<select name="course" onchange="if(this.value==-1){addop(this)}" >
   <option>select your course</option>
   <?php
      $query="SELECT Course_Name FROM courses";
      $result=mysqli_query($conn,$query);
      if($result)
      {
          while($row=mysqli_fetch_array($result)) {
              $Course_Name=$row["Course_Name"];

              echo "<option value='".$Course_Name."'>".$Course_Name." </option>";

          }
      }

      ?>
   <option value="-1">Add Another</option>
</select>
<script type="text/javascript">
   function addop(r)
   {
    var c = window.prompt("Add Custom Course");
    if(c!=null && c!= -1)
    {
        var str = '<option value="'+c+'">'+c+'</option>';
        r.innerHTML += str;
        r.value = c;
    }

   }
</script>