That flag is only supported in raw panels, not in the menu api. You can just add more text after a new line to the previous item like.
PHP Code:
Menu hMenu = new Menu(handler);
hMenu.SetTitle("First line\nsecond line");
hMenu.AddItem("1", "First item\nmore text on the next line before the second item");
hMenu.AddItem("2", "Second item\nmore text on the next line");