Untitled

<html>
<body>
<form action="http://example.com" method="get">
    User: <input type="text" name="username" /><br/>
    Password: <input type="password" name="password" /><br/>
    <input type="hidden" name="extra" value="lolcatz" />
    <input type="submit"/>
</form>
</body>
</html>