To embed the Mailchimp signup form to Squarespace, you can follow these steps
#1. When you finish creating the Mailchimp signup form, you will see the embed code.
#2. Edit page where you want to embed the Mailchimp signup form > Add a Code Block
and paste embed code
#4. Save and preview to see the result
#5. Note
To make it as Popup, when you create a form from Mailchimp, choose Popup form
Mailchimp will provide some code like this
Copy it and add to Code Injection > Header
result