WhatsApp doesn't support an official API. Because the data of WhatsApp remains on the device and not on a central server, it means that to have an API the API must also be live on the device, and then on the server, and then the requests should be sent down to the device and authentication and authorization and... Stop it! Long story short, they don't have an official API.
Many third-party custom libraries are available on the Internet, for C#, you may want to search on NuGet galleries and get a package. For example, this one,
https://www.nuget.org/packages/Whatsapp.NET/1.2.1[
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You can download the packages by executing the following command:
Install-Package Whatsapp.NET -Version 1.2.1
NuGet package manager would automatically download and store the .dll file in your project. You may keep that .dll file somewhere that you want to keep it. But that doesn't make any sense, either.