Get text from scene builder to create account screen

How do I get text from the FXML file?

FXML file field:

<TextField fx:id="usernameSU" GridPane.columnIndex="1" />

Controller file:

public class signUpController implements Initializable {

    @FXML private TextField usernameSU;

    @FXML
    private void signUp(ActionEvent event) {
        System.out.println(""); //Imprimir TextField para teste
    }

    @Override
    public void initialize(URL url, ResourceBundle rb) {
        usernameSU = new TextField();
    }    

}