app-module.ts

import { APP_INITIALIZER } from '@angular/core';

export function configServiceFactory (config: ConfigService) {
  return () => config.load()
}
...
@NgModule({
  ...
  providers: [
    ...
    ConfigService,
    {
      provide: APP_INITIALIZER,
      useFactory: configServiceFactory,
      deps: [ConfigService], 
      multi: true
    }
  ],
  ...
});