קוד:
<html>
<head>
<script>
function change()
{
document.getElementById('box2').value="#"+document.getElementById('box1').value;
}
</script>
</head>
<body>
<input id=box1 type=text onPropertyChange="change()" oninput="change()" >
<br>
<input id=box2 value='#' >
</body>
</html>