According to the Show HN guidelines[0], this is for things that are ready to be used, not coming soon / other sign-up only pages or blog posts:

> If your work isn't ready for people to try out yet, please don't do a Show HN. Once it's ready, come back and do it then.

> Blog posts, sign-up pages, and fundraisers can't be tried out, so they can't be Show HNs.

[0] https://news.ycombinator.com/showhn.html