Typically embed URLs have the following format:

https://glitch.com/embed/#!/embed/project-name?parameterName=parameterValue

However, on Medium you have to use the project URL as the embed URL, so it's format is like the following instead:

https://glitch.com/edit/#!/project-name?parameterName=parameterValue

So, to embed a project on Medium, select the add an embed option; then paste in the URL and hit enter.