auto login with cURL where submit button has no name ?

<form action="/dir/Twitter.aspx/InviteTweet" method="post">

                    <font face="Verdana" size="0">Usename: </font>
                    <input id="Text1" type="text" name="txtEmail" /><br />
                    <font face="Verdana" size="0">Password: </font>
                    <input id="Text2" type="password" name="txtPassword" /><br />
                    <br />
                    <input id="Submit1" type="submit" value="Sign in" />

HTML:

the input <input id="Submit1" type="submit" value="Sign in" /> has no name

my php code

$url = "http://www.my site.com/dir/Twitter.aspx/Index"; // This page contains Login form

        $postfields='txtEmail=username&txtPassword=password';
        $ch=curl_init($url);
        curl_setopt($ch,CURLOPT_RETURNTRANSFER,true);
        curl_setopt($ch,CURLOPT_FOLLOWLOCATION,true);
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $postfields);
        $page=curl_exec($ch);

        curl_close($ch);

PHP:

is there any way to use input id="Submit1" instead of it's name as there is no name ??