Hi,
postMessage(…) does not set the content of HTML component. It marelly posts the message to the component, that can be then received and acted upon.
You can do that with a bit of code inside of HTML. The code would receive the message and make it appear in the component. You can put this snippet into HTML component:
<script>
window.onmessage = (event) => {
document.getElementById("container").innerHTML = event.data;
}
</script>
<div id="container">
</div>
What it does, is listens to the message being posted and sets the content to the div container.
I hope this helps. Let me know!
You can also checkout the documentation here
And a community tutorial of a more complex case: