Thank you for reaching out to Constant Contact API Developer Support. It sounds like you are trying to use a 3rd-party app that is asking you for an API Key and/or an access token. It is most common to use our older V2 API for this.
Usually an application developer will generate their own API Key that lets their app communicate with Constant Contact, however some developers require you to provide them with your own API Key and/or Access Token. Since this process was designed for developers, it can be confusing for non-developers to find or generate this information. To help de-mystify the process for everyone, we have a blog post which walks through step-by-step instructions on how to get that information.
How to generate an API Key and Access Token:
If the app you are using does not give you a redirect URI in their documentation (and is only asking for an API Key and Access Token for example), you can just use the default value of:
Please have a look and let us know if you have any other questions!
Tier II API Support Engineer