Source code for https://victorfont.com/formidable-pro-invoke-ajax-submit/

invoke-ajax-submit.js

jQuery(document).ready(function ($) {
    "use strict";

    function set_form_fields(data) {
        $("#html_display_element").html(data);
        $("#field_confirm_value").attr("value", data ); // One way to set a value. Could also use $("#field_confirm_value").val( data );
    }

    $("#btnSubmit").on( "click", function (e) {
        e.preventDefault();
        var confirmation_req = $.ajax({
            type: 'GET',
            url: 'my_processing_script',
            dataType: "text",
            contentType: "text/xml",
            data: { FormXml: strXml },
            dataFilter: function (data, type) {
                var start = data.search('">') + 2, strlen = data.search('</long>') - start;
                return data.substr(start, strlen);
            }
        });
        confirmation_req.done(function(data) { set_form_fields(data); }); // fires on successful return of data
        confirmation_req.always(function() { $("#form_key").submit(); }); // always fires on completion of ajax request regardless of success or failure
    });
});