Billy,
Hi. Yes Dynamic Forms includes a feature called completion events and you can setup as many as you would like (or as few). These completion events can be setup to fire for any field response or based conditionally on a user response (i.e. add me to your email list checkbox or something).
Attached is a screenshot, basically you just need the URL you are posting to and also the parameters you are passing. So if Constant Contact is looking for a parameter of Email and First Name it would look like &Email=$(Email)&FirstName=$(FirstName). The tokens within $(Token) format represent the short field names on the forms.
You might also consider our Opt In Email 4.0 release which has many features of constant contact, but without service fees and also allows you to maintain all of your contact directly within DNN and not having to pay a monthly service.
-Chad