Bad Request on creating new Contact programmatically (Csharp)

Hi everybody,
i’m trying to post a Request to create a contact.
I’m following the suggest in Freshsales | Refreshingly new CRM & Deal Management Software
to create a code (cSharp) to be integrate in some automation (…).
The result is always a ‘Bad Request’.
others request (get) seem to succeed, but for post requests i have some issue.
What’s wrong, in your opinion?