Estava olhando e existe muita controvérsia, parece que tem uma problemas de autorização envolvidos.
Por favor, veja esses links:
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API Desenvolvimento WhatsApp para CSharp
https://social.msdn.microsoft.com/Forums/pt-BR/06667db1-5f46-4f7d-a75b-1ca2f0870a1d/api-desenvolvimento-whatsapp-para-csharp?forum=vscsharppt
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NuGet Package
Whatsapp API for .NET
This is a API written in C# but it can be used in any .NET language.
It's a fork from WhatsAPINet, which is based on WhatsAPI. The goal of this fork is to make it all work again and refacter the code, add documentation and write documentation on how Whatsapp Works.
https://nugetmusthaves.com/Package/WhatsAppAPI
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[]'s,
Fabio I.