I can post to my IIS server with c# but get a 405 error when posting to the same server with Javascript

I'm trying to post to the same server as my client on Windows 10. I can post from a c# program in the code behind but not to the same url using Javascript... which fails with a 405 Method Not Allowed error. The Javascript code looks like:

var xHttp = new XMLHttpRequest();
xHttp.open("POST", url);
xHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
xHttp.onload = function () {
    top.idxDisplayMessage(this.responseText);
};
xHttp.send(data);

Same error with jQuery:

function postHttpDoc(url, data) {
    // Data is in the proper object format here for jQuery
    var jqxhr = $j.post(url + '?' + data, function () {
        alert("success");
    })
        .done(function () {
            alert("second success");
        })
        .fail(function () {
            alert("error");
        })
        .always(function () {
            alert("finished");
        });
}

I've checked IIS's Handler Mappings, ExtensionlessUrlHandler-Integrated-4.0 and verbs say: GET,HEAD,POST,DEBUG,DELETE,PUT

I've added the following to web.config:

<system.webServer>
 <security>
    <requestFiltering>
        <verbs allowUnlisted="false">
            <add verb="GET" allowed="true" />
            <add verb="POST" allowed="true" />
            <add verb="DELETE" allowed="true" />
            <add verb="PUT" allowed="true" />
        </verbs>
    </requestFiltering>
</security>

But nothing has fixed the issue yet. What am I missing or not doing correctly?

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