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 generate an API key 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 for an API Key and Secret, then it most likely utilizes our V3 API. Your 3rd party integration may also provide you with the Redirect URI to add to your key. Follow steps 1-3 at the following link.: https://v3.developer.constantcontact.com/api_guide/apps_create.html
If the app you are using is asking you to also provide an “Access Token” with your API key, then it most likely utilizes our legacy V2 API, and you'd want to follow the steps at the following link instead:
*You will need to create a Mashery login for V2, as it does not use the same UN/PW as your Constant Contact account
The list ID format will be dependent on whether the script uses the legacy V2 API (10-digit list ID), or the current V3 API (32-digit list ID) version of our API, so you may need to ask them which format they need (or just generate both sets).
A lot of 3rd party integrations ask for customers to provide list IDs when setting them up, so we’ve made the following step-by-step article that shows regular users how to get both types of list IDs. All you’ll need is the credentials for your Constant Contact account and to follow the steps in the link below. Within the article, the first section is for our current V3 API, but if you scroll down, there are instructions for our legacy V2 API as well:
Obtain a List ID for APIs
Please have a look and let us know if you have any other questions!