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:

• Why I Don't like Email Newsletter Signup Pop Ups
  by Clint Watson
• Google will soon start punishing mobile sites that show hard-to-dismiss popups
• 25 reasons why I'll leave your website in 10 seconds
• Don't use annoying pop-ups
• 4 Reasons Why I Don’t Use Pop-Ups

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