HTML

<!DOCTYPE html>
<html>
<head>
    <title>Test</title>
    <meta charset="UTF-8"/>
    <script src="https://code.jquery.com/jquery-1.10.2.js"></script>
    <script>
    $(document).ready(function(){

        var feld = [2];
        feld[0] = EingabeText.value;
        feld[1] = EingabeText2.value;
        data = JSON.stringify(feld);

        var request = new XMLHttpRequest();
        request.open('post', 'Mailer.php', true);
        request.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
        request.send('json='+data);

        request.onreadystatechange = function() {
            if (request.readyState==4 && request.status==200){
                alert(request.responseText);
            };
        };

    });
    </script>
</head>
<body>
    <form id="bla" method="post">
        <input type="text" id="EingabeText" value="Testwert 1"/>
        <input type="text" id="EingabeText2" value="Testwert 2"/>
    </form>
</body>
</html>