Using a click listener when a button has have a settable ID

Your button ID

MyButtonId

Your handler

  <script>
    document.addEventListener("DOMContentLoaded", function(event) {
      document.getElementById('MyButtonId').addEventListener('click', function()
      {
        //Do something...

      });
    });
  </script>

Using a click listener when a button doesn’t have a settable ID

Your buttons link (its href=)

#MyButtonLink

Your handler

  <script>
    document.addEventListener("DOMContentLoaded", function()
    {
      document.addEventListener('click', function(Event)
      {
        const Button = Event.target.closest('a[href="#MyButtonLink"]');
        if (!Button)
          return;
        Event.preventDefault();

        //Do something...

      });
    });
  </script>