@NgModule({
  imports: [
    CommonModule
  ],
  declarations: [],
  exports: []
})
export class ScnApiModule {
  static forRoot(config?: IApiConfig): ModuleWithProviders {
    console.log('ScnApiModule', config); // here I get the config

    return {
      ngModule: ScnApiModule,
      providers: [
        {provide: 'ApiConfig', useValue: config},
        RestService,
...

@Injectable()
export class RestService {
  constructor(@Inject('ApiConfig') config?: IApiConfig) {
    console.log('RestService', config); // here I get `undefined`
...

export const CONFIG: IApiConfig = {
  apiHost: environment.scnApiLibrary.apiHost,
  authHost: environment.scnApiLibrary.authHost,
  clientId: environment.scnApiLibrary.clientId,
  secret: environment.scnApiLibrary.secret,
};

@NgModule({
...
imports: [
  ScnApiModule.forRoot(CONFIG)
]
...

@Injectable()
export class IApiConfig {
    apiHost:string;
    authHost:string;
    clientId:string;
    secret:string;

    constructor(init?: Partial<IApiConfig>) {
       this.apiHost = init.apiHost;
       ...
    }
}

export const apiConfigToken = new InjectionToken<IApiConfig>('apiConfigToken');

export function getapiConfigInjectionToken(config: IApiConfig) {
    return new IApiConfig(config);
}

export const APIConfigProvider = {
    provide: IApiConfig ,
    useFactory: getapiConfigInjectionToken,
    deps: [apiConfigToken]
};

export class ScnApiModule {
  static forRoot(config?: IApiConfig): ModuleWithProviders {

    return {
      ngModule: ScnApiModule,
      providers: [
         {provide: apiConfigToken, useValue: config},
            APIConfigProvider,
...

ScnApiModule.forRoot({apiHost: 123,authHost:456})