Button class is defined in videoosd.html
Button class is queried for in videoosd.js - view.querySelectorAll(".btnOsdFastForward")
Event listener is added to all found dom elements - onFastForwardButtonClick = fastForward(self, !0);

  function fastForward(instance, animate) {
    var player = instance.currentPlayer;
    animate &&
      (((animate = instance.view.querySelector(
        ".osd-ff-animationtext",
      )).innerHTML =
        "+" + parseInt(_usersettings.default.skipForwardLength() / 1e3)),
      fadeInAndOut(animate)),
      _playbackmanager.default.fastForward(player);
  }