Angular oidc client in unit test

I have an AuthentificationService service that calls OIDC client. I would like to test this service:

export class AuthentificationService {
  private manager: UserManager;
  private user: User = null;

  constructor() {
      this.manager = new UserManager(getClientSettings());

      this.manager.getUser().then(user => {
        this.user = user;
      });
  }

  completeAuthentication(): Promise<void> {
    console.log(this.manager);
    return this.manager.signinRedirectCallback().then((user) => {
      console.log(user);
      this.user = user;
    }).catch((err) => {
      console.log(err);
    });
  }
}

I want to test the completeAuthentication method but I can’t simulate the method. How to do it with a stub or mock?