<script type="text/javascript">
window.onload = function (e) {
var myHandler = function () {
var mytext ='Ваш текст';
document.getElementById('mytext').value = mytext ;
}

document.getElementById('butt').onClick = myHandler;
}
</script>
<input type="button" id="butt" value="Кнопка" />
<textarea id="mytext"></textarea>