index.html

<h1>Products catalog</h1>
<h2>Apple <button value="apple">Add to cart</button></h2>
<h2>Banana <button value="banana">Add to cart</button></h2>
<h2>Orange <button value="orange">Add to cart</button></h2>

<script>
  const addToCart = e => window.parent.postMessage(e.target.value, "*");
  document
    .querySelectorAll("button")
    .forEach(button => button.addEventListener("click", addToCart));
</script>