The website popup that collects newsletter subscribers is a separate feature from the campaign editor, configured from Marketing > Integrations > Email Newsletter Popup Form (Client Config tab Newsletter: Popup), and only appears when the Email Marketing module is active:

• Newsletter Popup - Title and Description – the heading and body text shown in the popup (the title supports the «client_name» placeholder).
• Newsletter Popup - Image – an image shown beside the form.
• Newsletter Popup - Fields – which of First Name, Last Name, Email and Phone are collected.
• Newsletter Popup - Show Behaviour – Show one time (even if not submitted successfully), Show every time pages loads, Show when triggered (Footer button click – a "Sign Up for our Newsletter" button in the site footer) or Don't Show.

A visitor who submits the form is matched by email against your registered customers (added straight to your newsletter list if matched) or, if no match is found, created as a new Contact and added to that list – the same handling used by the embedded Email Marketing Newsletter Form website module (Forms category, added to a page in the website builder). This also fires the User: New Newsletter event for flows. In the current codebase, which list a submission lands in is controlled by a backend-only setting tied to your account (there's no visible admin screen to pick or rename that list); if it hasn't been set up for your account, submissions fail with an error instead of saving. The popup remembers per-browser (via the visitor's local storage) whether it was already shown (for "Show one time") or submitted (for the other behaviours), so a visitor generally won't keep seeing it after their first visit or subscription.