<input type="number" id="miktar">

<select id="vergi" onchange="change()">
  <option value="8">%8</option>
  <option value="18">%18</option>
</select>

<input type="submit" onclick="hesapla()">

<p id="sonuc"></p>

<script>
    function change(){
      var vergiorani=document.getElementById("vergi").value;
      var miktar=document.getElementById("miktar").value;
      var vergi =miktar*vergiorani/100;
      document.getElementById("sonuc").innerText=vergi;
    }
    function hesapla(){
      var vergiorani=document.getElementById("vergi").value;
      var miktar=document.getElementById("miktar").value;
      var vergi =miktar*vergiorani/100;
      document.getElementById("sonuc").innerText=vergi;
    }
</script>