Finally got a Shiny app to work on a Squarespace page! Just use an embed block, click on the little double arrow button and insert this code:
<iframe width="100%" height="1000" src="shiny-app-url "> </iframe>
The key seems to be using iframe width as a percentage (it would NOT work when I tried to set the size of the iframe width to a number, like 1080).
Hope this works for you!