Embedding substack to squarespace site?

My site isn't live yet so I can't link, but I'd like to embed my substack link for people to subscribe to within my squarespace site. 

The embed URL I received from substack is as follows, but when I choose the "embed" option to insert it into my site, squarespace gives me a message "URL is not embeddable" lol. Can anyone help? : 

<iframe src="https://insidesources.substack.com/embed" width="480" height="320" style="border:1px solid #EEE; background:white;" frameborder="0" scrolling="no"></iframe>