Startup.cs

public partial class Startup
{
        public Startup(IHostingEnvironment env)
        {
            var builder = new ConfigurationBuilder()
                .SetBasePath(env.ContentRootPath)
                .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: false, reloadOnChange: true)
                .AddEnvironmentVariables();
            Configuration = builder.Build();
        }
        
        ....
}