Any time I have to embed code in Medium, I just type the code in a free Github Gist and post the link - Medium automatically embeds the Gist in a nice, syntax highlighted format, with a link to the original gist.

e.g. (towards bottom of this article I wrote - syntax highlighting in HTML, Javascript and Ruby snippets): https://hackernoon.com/building-a-face-recognition-web-app-i...