With the CCSFG, it is only possible to use Custom Fields that specifically have the names Custom Field 1-15 in your Constant Contact account. We do have plans to implement support for all Custom Fields through our API in the future, but these changes will not impact the CCSFG, as it was built on an older version of the API.
If there is any more information that I can provide, please let me know!
Elijah G. API Support Engineer
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