tampermonkey send url to local server

// ==UserScript==
// @name           Track Pageview
// @namespace      deKexx
// @description    Sends Url to a local server
// @include        http://www.youtube.com/watch*
// @include        https://www.youtube.com/watch*
// @connect        popo.local
// @author         Jan Zeiseweis
// @version        0.1
// @grant          GM_xmlhttpRequest
// ==/UserScript==
(function() {
    'use strict';
    var oldHref = location.href;

    function track() {
        var requestDetails = {
            method: "POST",
            url: "http://popo.local:5555/tracker/api/v1.0/pageview",
            data: `{"url":"${oldHref}"}`,
            headers: {
                "Content-Type": "application/json"
            },
            onload: function(response) {
                console.log(response.responseText);
            }
        }

        console.log(requestDetails);
        GM_xmlhttpRequest(requestDetails);
    };

    track();

    // Track pages that don't really reload, but refresh the entire content (like youtube).
    var mutationObserver = new MutationObserver(function(mutations) {
        mutations.forEach(function(mutation) {
            if (oldHref != document.location.href) {
                oldHref = document.location.href;
                track();
                return true;
            }
        });
    });

    mutationObserver.observe(document.getElementById('movie_player'), {
        attributes: false,
        characterData: false,
        childList: true,
        subtree: true,
        attributeOldValue: false,
        characterDataOldValue: false
    });

})();