Hello user286192,
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.
You do need to create the custom fields prior to being able to add data to them.
It looks like you are using the names of the custom fields under "custom_field_id" rather than the actual IDs of the custom fields.
You'd want to make a GET call to /contact_custom_fields in order to get the correct IDs of the custom fields:
GET custom_fields Collection
https://v3.developer.constantcontact.com/api_reference/index.html#!/Contacts_Custom_Fields/getCustomFields
Please have a look and let us know if you have any other questions!
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