Если кнопка это ссылка, то в атрибуты ссылки.
<div id="mk-button-11" class="mk-button-container _ relative mk-animate-element flip-x block text-center twf mk-in-viewport">
<a onclick="yaCounter39276445.reachGoal('buyclick');" href="#" target="_self" class="mk-button js-smooth-scroll mk-button--dimension-flat mk-button--size-large mk-button--corner-pointed text-color-dark _ relative text-center font-weight-700 no-backface letter-spacing-2 inline">
<span class="mk-button--text">ПОЛУЧИТЬ БИЛЕТ</span>
</a>
</div>
