requestToken method
Implementation
Future<Token> requestToken() async {
late Token token;
final String urlParams = _constructUrlParams();
String initialURL =
('${_authorizationRequest.url}?$urlParams').replaceAll(' ', '%20');
_webAuth(initialURL);
var jsonToken = await _onCode.first;
token = Token.fromJson(jsonToken);
await _messageSubscription?.cancel();
await _popupStatusSubscription?.cancel();
return token;
}