Result

The final result will look something like this

http://eepurl.com/gJOjFX

How to create the sign up form

1. Go to Mailchimp Dashboard

2. Click on Audience

3. Click on Signup forms

4. Click on Form builder

5. Copy the Signup form URL

6. Embed the form in Notion

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.