You have to use the Exclamation mark, square brackets, then round brackets. ( )

You only put text in the square brackets if doing a text link.
If you're referring to adding or changing the cover image of a story, you can do that on the Edit Story page.
To get there, go to your My Stories list. There, find your story, click on the vertical ellipsis and choose Edit Story.
The cover image doesn't have to be a link but can be uploaded.