Thank you for reaching out to Constant Contact API Developer Support, and for your patience. My team is here to assist outside software developers with questions about building into Constant Contact's API.
It looks like we've already followed up with you directly via email, but here is the information provided for visibility on the forum: We don't currently have an example project available in C#, however we can certainly review your code if you provide us with a sample via email at webservices@constantcontact.com
You also mentioned that you've tested using Postman. Postman does include the ability to generate code snippets (https://learning.postman.com/docs/sending-requests/generate-code-snippets), which may be useful, although I see that uses RestSharp rather than HttpClient for C#.
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