CCFSG PHP Signup Form Generator - this tool will create a simple HTML signup for that you could use in your popup window. It's free but does require you do know how to upload an PHP application to your wordpress installation and then configure it. Not overly advanced, but definitely will let you do what you're looking at.
Custom form designed using our API - this option would give you the ultimate flexibility into creating your own customized form. It also is the highest level of technical complexity because it requires some web development using an API. This is only recommended for web developers who are comfortable working with REST APIs and PHP scripting
Formstack Signup Widget - one of our partners offers a fully comtomizable signup form creator for building exactly these types of widgets. This is very easy to use, WYSIWYG form creator. This is a paid service outside of Constant Contact so that is the on caveat to be aware of here.
I hope one of these solutions will fit your needs.
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