You can find Mosspaper's Widget here: https://app.mosspaper.com/d/#/widget. There are 2 sizes available for you to embed into your web site.
1) Log in to SquareSpace.
2) Select the page you would like to have the widget displayed.
3) In Page content, click on "Edit".
4) Select a "Content block".
5) Click on "code".
6) Copy and Paste the iframe code into the box. Click on Save.