Hello TomP1,
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.
For a PUT call to /contacts, the contact ID should go in the request URL, rather than the request body.
You can check the request URL details and body schema in our API Tester at the link below. In the upper right where it says "BODY SAMPLE" you can expand the section to see an example of how the JSON request body should be formatted.
PUT (update) a Contact
https://v3.developer.constantcontact.com/api_reference/index.html#!/Contacts/putContact
Please have a look and let us know if you have any other questions!
If you continue to run into issues, feel free to reach our team directly/securely via email at webservices@constantcontact.com with your full request URL and JSON request body, referencing case #32589373.
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