Here is one way you can do it...
<body>
<textarea rows="5" cols="20" wrap="hard" onblur="CopyToClipboard(this)"></textarea>
</body>
<script language="JavaScript">
function CopyToClipboard(text) {
Copied = text.createTextRange();
Copied.execCommand("Copy");
}
</script>
This only works with IE 4 and above. When you run it, a dialog may come up asking you whether or not "you want this website to have access to your clipboard". Click yes if it does. Whatever text the user entered into the box will be copied to the clipboard.