I use MailerLite. I found the best way to give away freebies was to create the landing page in MailerLite and add the landing page link in my Squarespace button.
Of course I created a nice freebie design style on my SS site, added button and done.
I tried so many ways but didn't like the way the pop-ups looked.
As far as newsletter sign-ups go-the embed form works best on my sidebar.
I am currently working on figuring out how to get MailerLite to work with Squarespace's regular newsletter form. I'll keep on trying and update if I find a way or any other way of going about it.
Best of luck.
PS: If you'd like to take a peek at what I'm talking about please feel free at mariagagnon.com