You can see the contact ID--in our new API it is just labeled "ID" when it is returned in the JSON payload from a GET on a specific contact. One important thing to note is that the contact ID is read only, so you can pull and use it on your side, but you won't be able to change/specify a contact ID upon adding a new contact or updating an existing contact. Let us k now if you have any follow up questions!
Edit: For the purposes of email campaigns, there isn't a property tag (what the FAQ showed) for the contact ID. You could use the APi to pull back the contact ID from the contact API, however. For use with a custom link inside your campaign, the process would be far simpler if you used the email address property as the identifier though. Using the contact ID would involve storing the actual contact ID in another field, like one of the custom field links.
API Support Specialist
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