function callback(tp) {
        var txt = document.getElementById("txtContent");
        txt.value = txt.value + tp;
        txt.focus();
    }

<script type="text/javascript">
    var el = document.getElementById("Text1");
    var Range = el.createTextRange();
    Range.moveEnd('character', el.value.length);
    Range.moveStart('character', 0); //
    Range.collapse();
    Range.select();
</script>