Have a look at how TemplatePagesFeature or ServerEventsFeature uses appHost.RegisterService() to register services.
On a side-note I’d really recommend avoiding RequestScope, it’s tied to a Request Context and if your resources are pooled (like most of ServiceStack providers, e.g. OrmLite/Redis) then it’s worse for performance as it keeps resources open for longer. I never use it, if your dependency is ThreadSafe use AddSingleton if it’s not use AddTransient.