To get that HTML, open "view page source" or Dev Tools in your browser on a any Ghost blog page (you can use this one if you want, or your own site). Copy everything from (and including) <figure> to </figure>.

Microchic's Tech Blog
This box I created in the editor by pasting in the HTML used by the system to generate a bookmark card, and editing it.
This is some title text in a box styled as a bookmark card.
This is some plain text in a box styled as a bookmark card.