Thank you Mirek

Unfortunately the code always shows a hand.
Next option works, although it probably could be improved:

#include <CtrlLib/CtrlLib.h>

using namespace Upp;

struct App : TopWindow {
	bool showHand = false;
	
	virtual void LeftDown(Point, dword) 	{showHand = true;}
	
	virtual void LeftUp(Point, dword) 	{showHand = false;}
	
	virtual Image CursorImage(Point, dword) {
		if (showHand)
			return Image::Hand();
		else
			return Image::Arrow();
	}
};

GUI_APP_MAIN
{
	App().Run();	
}

struct App : TopWindow {
	virtual Image MouseEvent(int event, Point, int, dword) {
		if (event == CURSORIMAGE && GetMouseLeft()) 
			return Image::Hand();
		return Image::Arrow();
	}
};