Hello AlexM760,
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 get V3 API contact IDs, you’ll want to make a GET call to the Contacts endpoint:
GET https://api.cc.email/v3/contacts
If you are looking for the contact ID of a specific email address, you can add that as a parameter like so:
GET https://api.cc.email/v3/contacts?email=email@domain.com
You can make this call through your platform, or you can just use our V3 API tester which I have linked below. You'll need to be logged into the account that you are retrieving contact data from when using the tester.
V3 API tester: GET Contacts Collection https://v3.developer.constantcontact.com/api_reference/index.html#!/Contacts/getContacts
Please have a look and let us know if you have any other questions!
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