Angular 6 testing Component with User Session using Karma and Jasmine

I am just starting to configure my testing environment for my Angular 6 app. What's the best way for me to simulate a user logging in, so that my test passes? I have an AuthService, but only want to inject a mock of the service.

The line of code in the .ts file causing the issue is this.authService.userSession.authToken

and the error reads Cannot read property 'authToken' of undefined

Here's my edit-site.component.spec.ts file so far, where I attempt to create a mock auth service

import { NavigationComponent } from '../navigation/navigation.component';
import { EditSiteComponent } from './edit-site.component';
import { FormsModule } from '@angular/forms';
import { RouterTestingModule } from '@angular/router/testing';
import { MatDialog, MAT_DIALOG_SCROLL_STRATEGY } from '@angular/material';
import { Overlay } from '@angular/cdk/overlay';
import { HttpClient, HttpHandler } from '@angular/common/http';
import { InjectionToken } from '@angular/core';
import { AuthService } from '../../services/auth.service'
import { UserSession } from 'src/app/models/user-session';

describe('EditSiteComponent', () => {
  let component: EditSiteComponent;
  let fixture: ComponentFixture<EditSiteComponent>;

  class MockAuthService {
    isLoggedIn = true;
    user = { name: 'Test User'};
    authToken = '123456789'
  };

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      imports: [ FormsModule, RouterTestingModule ],
      declarations: [ EditSiteComponent, NavigationComponent ],
      providers: [{provide: AuthService, useValue: MockAuthService}, MatDialog, Overlay, HttpClient, HttpHandler, {provide: MAT_DIALOG_SCROLL_STRATEGY, useValue:{}}]
    })
    .compileComponents();
  }));

  beforeEach(() => {
    let userSession = MockAuthService
    fixture = TestBed.createComponent(EditSiteComponent);
    component = fixture.componentInstance;
    fixture.detectChanges();    
  });

  it('should create', () => {
    expect(component).toBeTruthy();
  });
});

In my auth.service.ts file is

public get userSession(): UserSession {
    return this._userSession;
  }

which references the model user-session.ts

import { ApplicationModel } from "./application-model";
import { SiteModel } from "./site-model";

export class UserSession {
    public _authToken: string;
    private _userContext: UserContext;
    public _currentApplication: ApplicationModel;

    constructor(authToken: string, userContext: UserContext) {
        this._authToken = authToken;
        this._userContext = userContext;
    }

    get authToken(): string {
        return this._authToken;
    }

    get userContext(): UserContext {
        return this._userContext;
    }

    get currentApplication(): ApplicationModel {
        return this._currentApplication;
    }

    set currentApplication(currentApplication: ApplicationModel) {
        this._currentApplication = currentApplication;
    }
}

Thank you so much!