Most of the desktop and mobile clients have the option of posting to public by default.

But I think your problem here is something else. You replied to a comment, instead of replying to the original post. Just reply to the original post. Your comment is totally public, as you can see.

Sharing a comment does not break the thread, by the way. I see you're using the web interface. It gives a very limited view of things. I suggest you try the desktop clients.