Hello Chris,
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 your request was formatted correctly, but the list that you are trying to post to (based on the example you provided) has been deleted, which is most likely what caused the 400 response.
Please have a look and let us know if you have any other questions!
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