There is an integration called Feedbolt for WordPress that does this automatically, but I doubt there is anything for SquareSpace. You could just send out an email broadcast manually to your list when you post a new blog post.
To get a web form on a SquareSpace personal page, you’ll have to edit the web form code to remove some scripts as SquareSpace does not allow
<script> tags on the personal plan.
- Create a campaign that starts with a web form that leads to a sequence. The sequence adds a “newsletter” tag (or whatever you want to name it.)
- Grab the web form code (unstyled HTML)
- Remove all the scripts at the bottom. but beware, this will open you up to bot spamming as you aren’t going to be able to use Google ReCAPTCHA.
Put the rest of the code into a code block on your page and you are good to go.
Now, when you post your blog, just send out an email broadcast to those you tagged “Newsletter” in your campaign.