Hi @photoon,
although having Fluent Forms Pro as well, I use Green Popups for this purprose. This plugin enables me to design both popups / forms AND customizable double-opt-in for each form (both text and e-mail can be set separately!). Once the confirmation link is clicked, the user is added to FluentCRM as “subscribed”. I also added a custom field to mark, which form led the subscriber to my database. Green Popups has integrated support for FluentCRM, I have modified the file to add custom fields and “full name” feature, though.
Another approach would be just to use a generic “e-mail confirmation” and a generic “[email protected]” e-mail address.
Best regards
Bart