User assigned managed identity for accessing key vault

Program.cs

public class Program
{
  public static void Main(string[] args)
  {
      CreateHostBuilder(args).Build().Run();
  }

  public static IHostBuilder CreateHostBuilder(string[] args) =>
      Host.CreateDefaultBuilder(args)
          .ConfigureAppConfiguration((context, config) =>
          {
              var keyVaultEndpoint = GetKeyVaultEndpoint();
              if (!string.IsNullOrEmpty(keyVaultEndpoint))
              {
                  // Get clientId of the user assigned managed identity
                  // Use it as AppId in below string
                  var connectionString = "RunAs=App;AppId=43b276c5-9441-4643-87e4-3807b9c3933e";
                  var azureServiceTokenProvider = new AzureServiceTokenProvider(connectionString);
                  var keyVaultClient = new KeyVaultClient(
                      new KeyVaultClient.AuthenticationCallback(
                          azureServiceTokenProvider.KeyVaultTokenCallback));
                  config.AddAzureKeyVault(keyVaultEndpoint, keyVaultClient, new DefaultKeyVaultSecretManager());
              }
          })
          .ConfigureWebHostDefaults(webBuilder =>
          {
              webBuilder.UseStartup<Startup>();
          });

  private static string GetKeyVaultEndpoint() => "https://<<your-key-vault>>.vault.azure.net";
}