From Xojo Documentation
Property (As MDIWindow )
| This property is only available on the Windows platform.
For cross-platform development, use ＃If...＃Endif with the Target... specifiers to make sure you will not attempt to use this property on an incompatible platform.
Provides access to the properties and methods of the MDIWindow class.
Valid only for Windows apps that are built using the Multiple Document Interface option.
This code sets the Title property of the app's MDI window.
App.MDIWindow.Title = "The MDI Window Title"