Returning textfield after buttonclick

I'm trying to move a string from one screen to another.

Once the program is started I create a screen with a button.

Clicking this button I create a new screen with a textField and a button.

I want the program to return what the user writes in the textfield after they click the second button.

I tried to put it inside the second buttons lambda, but that did not work

( e -> {
           	String name= ConfirmBox.register();
            return name;
        });

The second thing I noticed was that in my first buttons actionListener

button.setOnAction(e -> {
        String string= ConfirmBox.register();
        System.out.print(string);   
    });

I get the output null once i press the first button. Im guessing this is because the return is to quick, but how do I slow it down so that I get the correct return after the user have pressed the button?

public static void main(String[] args) {
		launch(args);
		
	}

	@Override
	public void start(Stage primaryStage) throws Exception {
		window = primaryStage;
		window.setTitle("titel");
	    button = new Button("button");
		
		button.setOnAction(e -> {
			String string= ConfirmBox.register();
	        System.out.print(string);	
		});
     }


 public class ConfirmBox {
     static String save; 

public static String register() {

	Stage window = new Stage();
	
	window.initModality(Modality.APPLICATION_MODAL);
	window.setTitle("title");
	
	Label label = new Label("enter tekst");
	TextField tekstField = new TextField();
	Button button = new Button("ok");
	
	
	
	button.setOnAction(e-> 
	{
        /*
        String name= ConfirmBox.register();
        return name;
        */

	save  = tekstField.getText();
	window.close();
	});

	GridPane layout = new GridPane();
	GridPane.setConstraints(label, 0, 0);
	GridPane.setConstraints(tekstField, 0, 1);
	GridPane.setConstraints(button, 0, 2);
		
	layout.getChildren().addAll(label, tekstField, button);
	Scene scene = new Scene (layout, 300, 250);
	window.setScene(scene);
	window.show();

	return save;
	  	
	}	
}

You are correct that the return statement is executed essentially immediately, so before the user has pressed the button, causing the String save to be set.

Instead of window.show(), use window.showAndWait(), which will block execution until the window is closed, achieving the desired result. Note that there's no real reason to have a variable for this at this point, you can just look up the value in the text field:

public class ConfirmBox {

    public static String register() {
    
        Stage window = new Stage();
        
        window.initModality(Modality.APPLICATION_MODAL);
        window.setTitle("title");
        
        Label label = new Label("enter tekst");
        TextField tekstField = new TextField();
        Button button = new Button("ok");
        
        
        
        button.setOnAction(e -> window.close());
    
        GridPane layout = new GridPane();
        GridPane.setConstraints(label, 0, 0);
        GridPane.setConstraints(tekstField, 0, 1);
        GridPane.setConstraints(button, 0, 2);
            
        layout.getChildren().addAll(label, tekstField, button);
        Scene scene = new Scene (layout, 300, 250);
        window.setScene(scene);
        
        //window.show();
 
        window.showAndWait();
    
        return tekstField.getText();
        
    }   
}

It's worth remarking here that you're reinventing the wheel to some degree. Have a look at the TextInputDialog class, and related classes such as Dialog and Alert.