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>

Try adding a Code Block > Paste your code

Site URL: https://porodicnekompanije.com

Hello!

I tried embedding Substack form by adding Code Block.

However, I received the following error message: 
JavaScript and iframe embeds aren’t supported in your plan. This code will not appear on your site.

Is there any way around this, other than upgrading my plan?

Thanks in advance!

What worked for me was adding an "embed block" and then clicking into the "</>" on the right of the URL text area and what pops up next is where you add the iframe.