How to add a Subscription box?
There are basicly two alternatives:


Using the Ctoosl Modal module. This module, included on the ctools project, is the most popular way to popup modal forms on Drupal, with lots of addon modules (like modal_forms). Here is a nice tutorial.
Using a custom module. You may find the following modules helpful: 


popup_message - which seems well supported. 
popup_announcement - this is a new module which seems closer to what you search for, with full html support, but with only a few downloads.  



With option 2, you should have to create the form and inject it into the popup using the forms API, so here is a form creation tutorial, and a stackexchange discussion about the best form creation method.