<table class="popup" style="position: absolute; background-color: black; border-radius: 10px; top: 50%; left: 50%;">

          <tr>

              <td>

                  <div>

                      <textarea style="margin:10px;  background-color:azure;  height:150px;width:100px;" contenteditable="true"></textarea>

                  </div>

              </td>

          </tr>

        </table>

   $(function () {

            var $popup = $('.popup');

            $popup.css({marginLeft: $popup.width() / 2 * -1 + 'px', marginTop: $popup.height() / 2 * -1 + 'px' });

        });