Абсолютно также:
<script type="text/javascript" language="javascript">
function you() {
var a = document.getElementsByTagName('input')[0];
var b = a.value;
alert(b);
}
</script>
<input class="loginField" type="password" name="password" size="20" style="width:100%" maxlength="15" onSubmit="return send(this);" /><br />
<input type="button" onclick="you();" value="Показать" />