document.getElementById("submit").addEventListener('click', function (e) {
    e.preventDefault();

    var req = new XMLHttpRequest();
    req.open('POST', 'https://httpbin.org/post', true);
    req.setRequestHeader('Content-Type', 'application/json');

    // On request complete, execute function
    req.onreadystatechange = function () {
        if (req.readyState == 4 && req.status == 200) {
            var res = JSON.parse(req.responseText);
            document.getElementById("res").value = res.data;
        }
    }

    var username = document.getElementById("username").value,
        password = document.getElementById("pass").value;

    var info = {
        user: username,
        pass: password
    };

    req.send(JSON.stringify(info));
});