You can easily embed your signup form on Medium by doing the following:
1. Copy your form URL
2. Paste into Medium story
3. Press ENTER (don’t forget this step!)
4. Wait 1 second…boom
If that doesn't work you can also use the + button on the left side of the editor and then select the <> embed button. A little text field will show up where you can paste you form URL and press enter.
NOTE: The form will show as an in image in edit mode but everyone who views the published version WILL be able to submit their email :).
Here is another nifty explanation…