[Keramet]

<table border='1'>
        <tr>
            <td id="in">Отсюда</td>
        </tr>
    </table>
    <form action="../ewfwfwf/fewfewfw" method="post">
        <input id='tag' name='tag' type='hidden' value='Сюда'>
    </form>
    <input type='button' id='btn' value='Перенести'>
    <script>
      document.querySelector("#btn").onclick = function() {
	     document.querySelector("#tag").value = document.querySelector("#in").textContent;
	}
    </script>