Hello World

API https://developers.google.com/youtube/v3/docs/subs...
Вот как реализована у vkmix (но там не через ajax, а делают так будто пользователь нажимает кнопки ):

subscribe: function subscribe() {
        var _selectors2;

        var selectors = (_selectors2 = {}, (0, _defineProperty3.default)(_selectors2, PREF_NEW, function () {
            return {
                done: (0, _jquery2.default)('#subscribe-button paper-button.ytd-subscribe-button-renderer').attr('subscribed') !== undefined,
                scroll: (0, _jquery2.default)('#contents'),
                loaded: '#comments',
                subscribe: (0, _jquery2.default)('#subscribe-button > ytd-subscribe-button-renderer > paper-button')
            };
        }), (0, _defineProperty3.default)(_selectors2, PREF_OLD, function () {
            var selector = '.branded-page-v2-col-container';

            return {
                done: (0, _jquery2.default)('.yt-uix-button-subscribed-branded.yt-uix-subscription-button').attr('data-is-subscribed') !== undefined,
                scroll: (0, _jquery2.default)(selector),
                loaded: selector,
                subscribe: (0, _jquery2.default)('.yt-uix-button-subscribe-branded.yt-uix-subscription-button')
            };
        }), _selectors2),
            selector = selectors[PREF]();
        var i = 0,
            cycle = {
            checkDone: function checkDone() {
                if (selector.done) {
                    worker.done();
                } else {
                    this.scrollContents();
                }
            },
            scrollContents: function scrollContents() {
                var _this2 = this;

                (0, _jquery2.default)('html, body').animate({
                    scrollTop: selector.scroll.offset().top
                }, TIMEOUT_CHANNEL.SCROLL).promise().then(function () {
                    interval = setInterval(_this2.contentsLoaded, TIMEOUT_SCROLL_BACK);
                });
            },
            contentsLoaded: function contentsLoaded() {
                i++;
                if ((0, _jquery2.default)(selector.loaded).length) {
                    clearInterval(interval);

                    (0, _jquery2.default)('html, body').animate({
                        scrollTop: 0
                    }, TIMEOUT_SCROLL_BACK).promise().then(function () {
                        setTimeout(cycle.subscribe, TIMEOUT_CHANNEL.TOTAL - TIMEOUT_CHANNEL.SCROLL - TIMEOUT_SCROLL_BACK * 2);
                    });
                } else if (i >= 1000 / TIMEOUT_SCROLL_BACK * 30) {
                    clearInterval(interval);
                    worker.ignored();
                }
            },
            subscribe: function subscribe() {
                selector.subscribe.click();

                setTimeout(worker.done, 3500);
            }
        };
}