How to add a Ghost subscription form to external site

Ghost blog:

We would like to use our Ghost blog for our newsletter group & signup. The current newsletter signup form on our front page is custom-made, including the member database. We’d like to transition this to Ghost, but I can’t find any documentation on how to create a signup form for an external site that isn’t using Ghost.

In other words, I’m looking for the Ghost equivalent of such an article: