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.
It looks like we've already reached out to you via email regarding this issue to request some additional information in order to assist with troubleshooting, but here is the information provided for visibility on the forum:
In regards to the 400 error, that's often due to malformed data or a data validation error. Just so we can research and test the issue further, would you be able to email us the body coding you're using to add those fields into Constant Contact?
If you haven't received our email reply, please let us know via email at email@example.com, and reference case number 27790342.
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