TrelloController.cs

public class TrelloController : Controller
{
    private string _apiKey = Environment.GetEnvironmentVariable("TRELLO_API_KEY");
    private string _token = Environment.GetEnvironmentVariable("TRELLO_TOKEN");

    private readonly HttpClient _httpClient;
    public TrelloController()
    {
        _httpClient = new HttpClient
        {
            BaseAddress = new Uri("https://api.trello.com/1/")
        };
    }
}