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You can edit the popup’s content in Theme Settings -> Shop -> Promo popup. You can create an HTML Block with Visual Composer and then use its shortcode for the popup.
Question: Is it possible to have the popup appear soon as I press the Newsletter button at the top bar ?
How is that possible without opening a page first and then press a button for the popup to appear?
Yes, you can open this popup clicking on the menu item but you will need to add the following CSS class to that link in Appearance -> Menus woodmart-open-newsletter