I’m afraid you can only make posts a whisper, and not topics. You also can’t toggle an existing whisper post to be visible, or vice versa (unless you install a theme component).
Looking at the screenshot, I think you may mean Toggle Unlisted - which would remove it from the topic lists for most users (TL4 can still see them) but would be viewable by anyone visiting the topic directly. It would also send out a notification to anyone Watching that category when the topic was created, so may not be as hidden as you may expect.
Creating a topic in a non-public category and then recategorising it when you want to make it public would be the simplest option.
You could also make use of the Shared Drafts feature (shared drafts category) to create a draft version and then publish it to the correct category when ready.