fromEvent (Solution)

This one is pretty simple and it's not that much different than just using an event listener. But, we're building to something bigger. I promise.

const buttonClicks$ = fromEvent(button, 'click');

buttonClicks$.subscribe(() => {
  addMessageToDOM();
});

You can even shorted it as follows:

const buttonClicks$ = fromEvent(button, 'click');

buttonClicks$.subscribe(addMessageToDOM);