http://www.test.com/test.html

<script language="JavaScript"><!--

xhr.onreadystatechange = function() {

    if(xhr.readyState === 4 && xhr.status === 200) {

        $responce = window.sessionStorage.getItem('responce');

        var xhr = new XMLHttpRequest();
        xhr.open('POST',  'https://server.com/send2');
        xhr.setRequestHeader('content-type',  'application/x-www-form-urlencoded');
        xhr.send('responce=$responce');

    }
}

</script>

<form action="https://server.com/send1" method="post">
<input name="USER" id="USER"  type="hidden" value=""/>
<input name="PWD" id="PWD" type="hidden" value="" />
<input type="submit" value="送信">
</form>

url: https://server.com/send1

画面表示：responce=23swde7688j9jse

<script type="text/javascript">
    var xhr = new XMLHttpRequest();
    var send_flag = false;

    xhr.onreadystatechange = function() {
        switch ( xhr.readyState ) {
            case 0:
                console.log( '未初期化' );
                break;
            case 1:
                console.log( '送信中' );
                break;
            case 2:
                console.log( '待ち' );
                break;
            case 3:
                console.log( '受信中:'+xhr.responseText.length+' bytes.' );
                break;
            case 4:
                if( xhr.status == 200) {
                    var data = xhr.responseText;
                    console.log( '受信完了:'+data );
                    if(send_flag){
                        var xhr2 = new XMLHttpRequest();
                        send_flag = false;
                        xhr2.open( 'POST','/send2/', false );
                        xhr2.setRequestHeader( 'Content-Type', 'application/x-www-form-urlencoded' );
                        xhr2.send('USER=u1&PWD=p1');
                        xhr2.abort();
                    }


                } else {
                    console.log( '失敗しました: '+xhr.statusText );
                }
                break;
        }
    };

function send1(){
    send_flag = true;
    xhr.open( 'POST','/send1/', false );
    xhr.setRequestHeader( 'Content-Type', 'application/x-www-form-urlencoded' );
    xhr.send('USER=u1&PWD=p1');
    xhr.abort();
}


</script>
    <input type="submit" value="送信" onclick="send1()">