Angular2+: How to use a service in a library

Question

Maybe I'm just plain stupid and do not understand what enno.void is referring to - or the question isn't really answered.

Regardless: I have a similar problem:
There is an existing app which I want my new library integrated in.
I know, there is a specific service in app which I need in library. For type purposes, I've created an interface in library. But how can I use this fancy service app in library?

---

Following was my initial approach:

app.module.ts (the existing app)

import { MyLibModule } from 'my-lib';
import { FancyService } from './fancy.service';

...

imports: [
    BrowserModule,
    MyLibModule.forRoot(new FancyService()),
],
providers: [
  FancyService
],

lib.module.ts (the new library)

import { FancyService } from './fancy-interface.service';

@NgModule({
  declarations: [ MyLibComponent ],
  exports: [ MyLibComponent ],
  schemas: [ CUSTOM_ELEMENTS_SCHEMA ]
})
export class MyLibModule
{
  public static forRoot(fancyService: FancyService):
    ModuleWithProviders<MyLibModule>
  {
    return {
      ngModule: MyLibModule,
      providers: [
        { provide: 'FancyService', useValue: fancyService}
      ]
    };
  }
}

---

That is working so far, but as you can see, I have to instantiate the requested service. Not cool. I wanted the same instance - not different ones.

As far as I've understood, you can only use that forRoot-method (or InjectionToken) if you have static values (which makes sense).

So my question is: how can I use FancyService with DI in my-lib?

Another approach was to use the Injector in the my-lib.component.ts

---

public constructor(private injector: Injector)
{
  this.fancyService = injector.get('FancyService');
}

---

As expected, an NullInjectorError was thrown. But I don't really have another clue. Any ideas?