Your HTML should look like:
<!--html above-->
<input type='password' id='pass' name='pass' value='yourPasswordHere' />
<!--more html below-->
<script type='text/javascript' src='common.js'></script>
</body>
</html>
You external JavaScript, saved as common.js
should look something like:
function E(e){
return document.getElementById(e);
}
E('pass').select();