Program.cs

public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
            WebHost.CreateDefaultBuilder(args)
                .ConfigureAppConfiguration((ctx, config) => {
                  
                    // Fetch app settings from Azure Key Vault if its in production mode.
                    if (ctx.HostingEnvironment.IsProduction())
                    {
                        var configRoot = config.Build();
                        var tokenProvider = new AzureServiceTokenProvider();

                        config.AddAzureKeyVault(
                            $"https://{configRoot["AppSettings:KeyVaultName"]}.vault.azure.net/",
                            new KeyVaultClient(new KeyVaultClient.AuthenticationCallback(tokenProvider.KeyVaultTokenCallback)),
                            new DefaultKeyVaultSecretManager());
                    }
                })
                .UseStartup<Startup>();