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.
To retrieve the Contact IDs, you'd want to make a GET call to the Contacts endpoint. You can search by individual email address, by status, list, and other factors. The response body of this call will include the contact_id for each contact returned:
GET Contacts Collection
Overview of V3 contacts functionality
You can utilize our API Reference Page to find body schemas & values, as well as test the functionality of our available endpoints to see how they will work once programmed within your application:
Please have a look and let us know if you have any other questions!