HttpClient POST request using x-www-form-urlencoded

You’re posting JSON data to the API instead of form data.
The snippet below should work.

login(username, password): Observable<any> {
  const body = new HttpParams()
    .set('username', username)
    .set('password', password);

  return this.http.post('/login',
    body.toString(),
    {
      headers: new HttpHeaders()
        .set('Content-Type', 'application/x-www-form-urlencoded')
    }
  );
}