<form id="..." action="..." method="post" autocomplete="off">

<script type="text/javascript">
        function Clicked(sender, args) {
            var textBoxNewValue = $find("<%= textPassword.ClientID %>").get_value();

            $find("<%= buttonLogIn.ClientID %>").set_value(textBoxNewValue); 

            $find("<%= textPassword.ClientID %>").clear();
            return true;
        }
    </script>