In your application initialization, set your API key and organization URL:
PipedriveClient client = new PipedriveClient(new ProductHeaderValue("PipedriveExample"), "[your organization url here]", "[your api key here]");
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.