FASO does not offer a way to add a pop-up to your website. We feel that most people get annoyed by these pop-ups.
Your site visitor may also have pop-up blockers installed, preventing the popups from appearing.
For more info on why not to use Pop Ups, please see these articles:
If you still wish to use a popup, you can use an off-site newsletter service like MailChimp and add their popup.
For more info, see this FAQ:
Mailchimp Integration / Embedded Form ~ Subscriber Pop-Up
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