Can I add a third-party newsletter sign up box?

Pretty much any email marketing platform can be integrated with Squarespace. If your platform doesn't directly integrate with Squarespace's Newsletter Blocks, you can always insert a Code Block and paste in your newsletter sign-up embed code.

Alternatively, you can connect a Squarespace Form Block to any email marketing platform using Zapier. More on that here: https://support.squarespace.com/hc/en-us/articles/360000641507
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