Howto show the topic id in Metadata of initial Post

[Falco]

Add this to Settings > Customize > Common > </head>:

<script type="text/discourse-plugin" version="0.5">
  api.decorateWidget('post-meta-data:after', dec => {
    if (dec.attrs.post_number === 1) {
      const topic = dec.getModel().get('topic');
      return dec.rawHtml(`<div class="post-info topic-id">Topic: ${topic.get('id')}</div>`);
    }
  });
</script>

And you get:

And you just need to add some CSS.