Callback adds new field to a form and...

Callback adds new field to a form and...

Hello,
I have a problem. I have a form where I add a field on ajax's call back but when I submit the form it won't the new field's value:

    $(document).ready(function() {
        $("#submit").on('click', function(){
           
            var datastring = $("#listingForm").serialize();
           
            $.ajax({
                type: "POST",
                url: "/member/submit_listing/",
                data: datastring,
                success: function(data) {
                    console.log(data);
                },
                error: function(){
                      alert('error handing here');
                }
            });
            return false;
        });
       
        $( "#parent_cat" ).change(function() {
           
            console.log(this.value);
           
            var form_data = {paren_cat: this.value};
           
            $.ajax({
                type: "POST",
                url: "/member/params/",
                data: form_data,
                success: function(data) {
                    if(data!="1") $("#params_div").html(data);
                },
                error: function(){
                      alert('error handing here');
                }
            });
        });
Please help!
Thank you!

Link to the site: http://ads.philippn.com/member/listing_form