To embed a signup form for your Substack publication head to your Settings page and scroll to "Import your email list".
Insert this HTML code onto Notion /embed block to embed a signup form for your Substack publication.
The embedded form will look like this
The embedded form will also support dark mode, inverting the colors of the embedded form when the color mode is dark.
Remove the scroll bar
To make sure there is no scroll bar make the block height as tall as you can, this way the iframe block will always have space and no scroll bar will show up.