how i can get and id of an element in combobox javafx

I add a ComboBox which I add in it entities from a database. I want to get the ID of the selected item in ComboBox, thanks.

public void initialize(URL url, ResourceBundle rb) {

    List <String> offres = new ArrayList <String> ();
    GestionOffre o = new GestionOffre();
    for (Offre i : o.afficherOffresEtablissement(1)){
        offres.add(i.getTitre());
    }
    OffreRemise.getItems().addAll(offres);
    // TODO
}  

@FXML
private void ajouterOfre(ActionEvent event) {

   String titre= TitreRemise.getText();
   String desc=DescRemise.getText();
   int  pourcentage =Integer.parseInt(PourcentRemise.getText()) ;
   Date dateDeb = Date.valueOf(DateDeb.getValue());
   Date dateFin = Date.valueOf(DateFin.getValue());
   String idoffre = OffreRemise.getSelectionModel().getSelectedItem().toString();
    Remise o = new Remise();
    GestionRemise r = new GestionRemise();
    r.ajouterRemise(o);


}