This is something that Flarum should have come with by default. I found an extension for the forum that allows for thread previews. I'm suggesting this because it is impossible to preview with formatting what you're writing which has led to a few moments where I've accidentally posted things such as a senior admin application I was drafting for later use in the wrong place because only replying to a post shows a preview of what you are writing, and hitting the wrong button can post it.
The extension is located here.
Here's an image from the extension's page showing what it looks like.