Here is a code to make window like that.

main.java@Override
public void start(Stage stage)
{
	StackPane root = new StackPane();
	// scene shape
	SVGPath p = new SVGPath();
	p.setContent("M0,0 l180,-150 a60,60 0 0,1 60,60 v180 a60,60 0 0,1 -60,60  h-180 a60,60 0 0,1 -60,-60 l0,-180  z");
	p.setFill(new Color(0.90,1,1,1));
	p.setEffect(new DropShadow());
	
	// form
	GridPane pane = null;
	try
	{
		pane = FXMLLoader.load(getClass().getResource("/resources/fxml_example.fxml"));
	} catch (IOException e)
	{
		e.printStackTrace();
		return;
	}
	
	// image
	Image image = new Image(getClass().getResourceAsStream("/resources/duke.png"));
	ImageView imageView = new ImageView(image);
	imageView.setScaleX(0.75);
	imageView.setScaleY(0.75);
	imageView.setTranslateY(-70);
	root.getChildren().add(p);
	root.getChildren().add(imageView);
	root.getChildren().add(pane);
	
	// transparent scene and stage
	Scene scene = null;
	scene = new Scene(root, 400, 400, Color.TRANSPARENT);
	stage.setScene(scene);
	stage.initStyle(StageStyle.TRANSPARENT);
	stage.setTitle("Custom Window");
	stage.show();
}

resources/fxml_example.fxml<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.geometry.*?>
<?import javafx.scene.layout.*?>
<?import javafx.scene.control.*?>
<?import javafx.scene.text.*?>


<GridPane 
    xmlns:fx="http://javafx.com/fxml" alignment="center" hgap="10" vgap="10"
    style=" -fx-font-family: sans-serif; -fx-focus-color: #99aaaa;">
    <padding><Insets top="150" /></padding>
    
	<Text text="Добро пожаловать" 
        GridPane.columnIndex="0" GridPane.rowIndex="0"
        GridPane.columnSpan="2"
        style="-fx-font-size: 16pt;" >
    </Text>
 
    <Label text="Логин:"
        GridPane.columnIndex="0" GridPane.rowIndex="1"/>
 
    <TextField 
        GridPane.columnIndex="1" GridPane.rowIndex="1"/>
 
    <Label text="Пароль:"
        GridPane.columnIndex="0" GridPane.rowIndex="2"/>
 
    <PasswordField fx:id="passwordField" 
        GridPane.columnIndex="1" GridPane.rowIndex="2"/>
        
</GridPane>