Thank you for reaching out to Constant Contact's API Support.
The use of custom fields can be a bit tricky depending on if they were created with the API or created in the website UI. Our API was created when the custom field names could only be named Custom Field 1 through 15. There is no endpoint to retrieve the list of custom fields as there is such a small amount of possibilities it was not needed at the time.
If you created custom fields in the website UI you are not limited for the names of the fields; however those field names will not work with our current API. We are developing v3 of our API which will allow the use of different names for the custom fields and we will make sure that is announced when it is fully released. Until then GETting a contact and reading the custom fields is the only way to see that data.
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