auth.component.ts

ngSubmit() {
  const username = this.form.get('username').value;
  const password = this.form.get('password').value;
  this.authService.authenticate(username, password).subscribe(() => {
    const url = this.returnUrl || '/';
    this.router.navigate([url]);
  }, (error) => {
    this.form.setErrors({
      notAuthorized: true
    });
  });
}