(The program has not been publicly rolled out yet. Only DGM-2271 students and other testers will be able to use it until the official rollout. Because some things might have changed, please leave an anonymous comment if you find anything.)
Medium uses Embed.ly as its primary method for embedding. This means that whether you want to embed a form, a photo or a rich document from another site, you need only enter the URL. Pasting in the embed code is not only unnecessary, it won’t even work.