Card apps1 are only available in the premium plan.
Using this type of apps you can embed an iframe or an App Card inside a post in the sidebar, beginning of the post, or at the end. You can also embed Cards inside user's feed.
Iframe cards are the easiest way to test this feature. In case you want to test it before upgrading to premium you can send the url to the page that you want to embed and where you want it to appear to firstname.lastname@example.org.