In your application initialization, set your API key and organization URL:

PipedriveClient client = new PipedriveClient(new ProductHeaderValue("PipedriveExample"), new Uri("[your organization url here]"))
{
  Credentials = new Credentials("[your api key here]", AuthenticationType.ApiToken)
};

You can obtain your secret API key from the API Settings https://[your organization].pipedrive.com/settings#api in Pipedrive.

You can debug this library right from your application by configuring the NuGet symbol server.