To create a pop up for any of the Shopify free themes, Brooklyn included, you'll need to use a third-party app from The Shopify App Store. There a few apps available which you can view here, but I'll list my top three recommendations below:
1. Privy ‑ Pop Ups, Email, & SMS - This is one of the more popular pop-up apps, and you can easily see with its 24405 reviews!
2. Pop Up, Email & Exit Pop Ups by Poptin - My personal favourite. It's so easy to use.
3. Original Wheelio spin pop‑ups by IT Trading d.o.o. - if you're looking for something a bit different, this is it! It does cost to use though, from $14.92 USD per month.
I hope this helps!
Thanks for your help. Ive also found this one which is free and will integrate visitor details directly into my shopify customer list which means I can use Shopify email as my single source of EDMs https://apps.shopify.com/powr-popup