index.html

<!-- import jQuery using the google CDN -->
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>

<script>
  $("button.add#1").on("click", function () {
    console.log("clicked button 1");
  })
</script>

<button class="add" id="1">Button 1</button>  
<button class="add" id="2">Button 2</button>
<button class="add" id="3">Button 3</button>