Install-Package Microsoft.AspNetCore.Cors

public void ConfigureServices(IServiceCollection services)
{
  services.AddCors();
}

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory, IConfiguration config)
{
  //  ...
  app.UseCors(options =>
      options.WithOrigins("http://example.com")
          .AllowAnyMethod()
          .AllowAnyHeader()
          .AllowCredentials();
  );
  app.UseCors(builder => {
      var allowedOrigins = config.GetSection("AllowedOrigins").GetChildren().ToArray().Select(c => c.Value).ToArray();
      logger.LogInformation("Allowed Origins: {0}", string.Join(", ", allowedOrigins));
      builder.WithOrigins(allowedOrigins).AllowAnyMethod();
  });
  //    ...
}