How to secure hardcoded fields in flutter

Question

I need to make targetEmail, username, password secure with the flutter_secure_storage package but I do not know how to do it.

class ContactBloc extends Bloc<ContactEvent, ContactState> {
  late final SmtpServer smtpServer;
  final String targetEmail = 'text1';
  final String username = 'text2';
  final String password = 'text3';

  ContactBloc() : super(ContactInitial()) {