可见微软文档标题栏自定义 (opens new window)
按照WinUI3的教程走就行

// MainPage.cs

IntPtr hWnd = WinRT.Interop.WindowNative.GetWindowHandle(App.MainWindow);
//如果在App.cs文件里则将App.MainWindow改为MainWindow
Microsoft.UI.WindowId myWndId = Microsoft.UI.Win32Interop.GetWindowIdFromWindow(hWnd);
Microsoft.UI.Windowing.AppWindow? appWindow = Microsoft.UI.Windowing.AppWindow.GetFromWindowId(myWndId);
Microsoft.UI.Windowing.AppWindowTitleBar? titleBar = appWindow.TitleBar;
titleBar.ExtendsContentIntoTitleBar = true;
titleBar.ButtonBackgroundColor = Microsoft.UI.Colors.Transparent;