jquery – Concatenate 2 JSON values and serialize JavaScript

In my webform I have an input field and I want to take input from the user and concatenate with my onw value like user input 123, my value test and the final result that would be send with the POST to be test123. What should I do, how to do this?

<form>
<input type="text" id="test" name="test">
</form>

AJAX

$(document).ready(function() {
    if ($("#formid").length) {
        $("#formid").submit(function(event) {
            var form = $(this);
            var url = form.attr('action');
            $.ajax({
                type: "POST",
                url: url,
                data: form.serialize(),
                //success...