function addRow()
 {
    tabBody=document.getElementById("inputtable");
    row=document.createElement("TR");
    cell1 = document.createElement("TD");
    textnode1=document.createElement("input");
    textnode1.name="inputbox[]";
    textnode1.type="text";

    cell1.appendChild(textnode1);
    row.appendChild(cell1);
    tabBody.appendChild(row);
}

<form name="" action="" method="post">
  <table border="0" width="100%">
    <tbody id="inputtable">
      <tr><td><input type="text" name="inputbox[]" /></td></tr>
    </tbody>
  </table>

  <table border="0" width="100%">
    <tr>
   <td><input type="button" value="Add a Question" onclick="addRow();" /></td>
    </tr>

 </table>
</form>