public ComboBox<Person> aComboBox;

aComboBox.getItems().addAll(People.getPeople());
aComboBox.getItems().add(new Person("Add New...", ""))

public void aComboBox() throws IOException {
    if (aComboBox.getValue().getFirstname() == "Add New...") {
            Parent AnotherView= FXMLLoader.load(getClass().getResource("AnotherView.fxml"));
            Stage Window = new Stage();
            Window.initModality(Modality.APPLICATION_MODAL);
            Window.setScene(new Scene(BillingView));
            Window.show();
    }
}

public void aComboBox() throws IOException {
if (aComboBox.getValue().getFirstname() == "Add New...") {
        aComboBox.setValue(null);
Parent AnotherView= FXMLLoader.load(getClass().getResource("AnotherView.fxml"));
        Stage Window = new Stage();
        Window.initModality(Modality.APPLICATION_MODAL);
        Window.setScene(new Scene(BillingView));
        Window.show();
}
}