Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
It sounds like you are trying to use a 3rd-party app that is asking you for an API Key. Usually an application developer will generate their own API Key that lets their app integrate with Constant Contact, however some developers require you to provide them with an API Key. Since this process was designed to be done by the app developers themselves, it can be confusing for end users to find or generate this information.
We have two different versions of our API, so the steps you take to get this information will depend on what version of our API the integration you are looking to connect with uses as they are created through different platforms and are not cross compatible.
If the app you are using is asking you to provide an “Access Token” with your key, then it probably utilizes our legacy V2 API, rather than our newer V3 API.