From the FAQs:
How do I make use of the new option to AJAXify the form?
Next you need to decide if you want the link to be on a WordPress page or in your Sidebar with the Widget.
For a WordPress page you use the normal Subscribe2 token but add a 'link' parameter with the text you'd like your users to click, so something like:
[subscribe2 link="Click Here to Subscribe"]
For Sidebar users, visit the Widgets page and look in the Subscribe2 Widget, there is a new option at the bottom called "Show as link". If you choose this a link will be placed in your sidebar that displays the form when clicked.