I finally figured it out! Yes, you have to use the Newsletter block, (which is where I got lost), and then connect that to a contact list. Then set up an automation so that when people are added to that list, it sends them an email with your freebie.
I went round and round with this until I found the answer... there should definitely be a video.
Here's the incredibly helpful tutorial I found:
https://www.firtherdesignco.com/blog/how-to-make-a-lead-magnet-on-squarespace
Good luck, freebie friend!