Accessing DOMelements generated by Shopify BuyButton.js UI Client

I’m building a website where I want to run a secondary function when the “Add to Cart” button is clicked.

For some reason when I try using any sort of event listener or getElementsbyTagName to access the button created by the BuyButton script, it always returns as undefined and gives me this error:

Uncaught TypeError: Cannot set properties of undefined (setting ‘innerHTML’)

Here’s my code:

   <script src="http://sdks.shopifycdn.com/buy-button/1.0.0/buybutton.js" ></script>    
</head>
<body>
    <div id="shopifyDiv"></div>


    <script type="text/javascript">
    /*<![CDATA[*/
        var client = ShopifyBuy.buildClient({
          domain: 'shopify.myshopify.com',
          storefrontAccessToken: '(myTokenHere)',
        });

        var ui = ShopifyBuy.UI.init(client);

        
        ui.createComponent('product', {
        id: (myIdHere),
        node: document.getElementById('shopifyDiv'),
        options: {
            product:{
              width: '350px',
              contents:{
                img: false,
                button: true,
                title: false
              },
              text: {
          button: 'Add to cart'
        },
            },
          cart:{
            startOpen: false
          },

        }
      
      });


    /*]]>*/
    </script>



  <script type="module">

document.getElementsByTagName('button')[0].innerHTML = 'hello world';

       </script>
    
</body>
</html>

I removed the Shopify specific accessToken & domain name for security purposes.

Any help in understanding why these DOM Elements that are being created by the Shopify script are inaccessible would be highly appreciated.