How to obtain checked checkbox values on the serverside in c# from an ajax Http POST using web forms (not MVC)?

Here's my ajax call:

$(function () {

        $("#chkFilter").on("click", "input", function (e) 
        {
            var filterCheckboxes = new Array();
            $("#chkFilter").find("input:checked").each(function () {
                //console.log($(this).val()); //works fine
                filterCheckboxes.push($(this).prop("name") + "=" + $(this).val());
                console.log($(this).prop("name") + "=" + $(this).val());

                //var filterCheckboxes = new Array();
                //for (var i = 0; i < e.length; i++) {
                //    if (e[i].checked)
                //        filterCheckboxes.push(e[i].value);
                //}
            });
        console.log("calling ajax");
        $.ajax({
            url: "/tools/oppy/Default",
            type: "POST",
            dataType: "json",
            data: { filterValues: filterCheckboxes }, // using the parameter name
            success: function (result) {
                if (result.success) {
                }
                else {
                }
            }
        });
        });
    });

And my server side code:

public partial class tools_oppy_Default : System.Web.UI.Page
{
    ...

    protected void Page_Load(object sender, EventArgs e)
    {
        if (Request.HttpMethod == "POST")
        {
            string checkedBoxes = Request["filterValues"];
            testLabel.Text = checkedBoxes;

        }

I'm just trying to obtain the post URL with the appropriate checked values so I can parse it on the server. However, I'm having trouble obtaining the URL. The string checkedBoxes is supposed to hold a query string like name=value&name=value&name.... but when I test it, the testLabel doesn't show anything. I'm using web forms app, not MVC. Also, I'm new to ajax and their behavior. Thanks.