Using FormKeep, how do I set the Accept header of the request to application/javascript

I'm using FormKeep http://ift.tt/1uuJqVb, and when I submit the form it redirects to a thank you page. But I want it to stay on the page and submit via AJAX. Below is my code, but in order to disable the redirect, they say I have to set the Accept header of the request to application/javascript.

How is this supposed to be done?

Can I submit forms with AJAX?

Yes, FormKeep accepts Cross Origin Requests. To disable the redirect, set the Accept header of the request to application/javascript.

//Simple form
$("form").submit(function(e) {
    e.preventDefault();

    $.ajax({
        type: 'POST',
        url: 'http://ift.tt/1bH4GVQ',
        data: $(this).serialize(),
        success: function() {
            //window.location = "http://google.com";
            alert('Success!');
        },
        error: function() {
            alert('Error!');
        }
    });
});

// ---------------------------------

// A more advanced form, with success message
var form = $("form.ajax");
form.submit(function(e) {
    e.preventDefault();

    if ($("body").hasClass("support")) {
        var formkeepID = "MyID"; //support
        var successMsg = " You're in the front of the line! You should be hearing from us soon. ";
    } else if ($("body").hasClass("partner-with-us")) {
        var formkeepID = "MyID"; //partner-with-us
        var successMsg = "Thank you! We'll get back to you shortly on partnering with us";
    }

    $.ajax({
        type: 'POST',
        url: 'https://formkeep.com/f/' + formkeepID,
        data: $(this).serialize(),
        success: function() {
            form.find("input, button, textarea").prop('disabled', true).css({"opacity": 0.5});
            form.find(".alert-success").text(successMsg).fadeIn(500);
        },
        error: function() {
            alert('Error!');
        }
    });
});