Hey, there !
Katy here from Shopify. Thanks for posting. ?
You can add this to your theme via the theme customizer. If you log into your admin and go to your:
Online store > Themes > Customize theme> Sections and click on the Footer section. Here you can add content and select 'newsletter'. This should then appear in your footer across all areas of your store.
We have a guide here that goes through these steps visually too.
I hope this helps- do feel free to let me know.