How to register service in the startup.cs file.

Hi
I have implemented the one of setting services below

public class SettingsService : ISettingsService
{
}

and I did dependency injection like the below 

public void ConfigureServices(IServiceCollection services)
{
            services.AddSingleton<ISettingsService,SettingsService>();
            services.AddTransient<ISettingsService,SettingsService>();
}

but it's not calling the  SettingsService.

Could you please suggest to us how can we register the service in the Optimizely CMS 12 and .netcore 6.0