How to integrate the Ninja Popups plugin with the Subscribe by Email plugin

I am using the Ninja Popups plugin from CodeCanyon on my clients website.

I am also using the Subscribe by Email plugin from WPMU.

Now, I want to integrate the Ninja Popups plugin with Subscribe by Email plugin, so that whenever a user enters an email in the Ninja popup window, then I will be added directly to the database with subscribers.

The Ninja Popups plugin has a Settings page where you can specify where the email is send to. There is a an option named "HTML Form", I believe this is the option that should be used but I am not quite sure how.

Please see attached image!

I would appreciate an help I can get on this issue!

Thanks a lot!
Sincerely,
Mika