How do I post a snippet to a slack thread?

You are not directly shown the option to post a snippet or an image in a thread. But there is a workaround for this. Steps

• Post the snippet/image in the channel or DM (I DM myself for this and delete it later)
• Copy Link to snippet/image
• Post the link in the desired thread. Slack shows a preview to the snippet/image in the thread itself as if it has been posted in the channel/DM as a normal snippet/image

Hope that helps!

According to this post you can't:

You can’t add images (we post screenshots within conversations ALL THE TIME), run any slash commands, or add code snippets.

It's sad really... I use threads a lot but they have some usability concerns.

As of today, the latest MacOS desktop version still doesn't allow you to post snippets to threads. I'm currently just using the web version which does support that.