From Xojo Documentation
| This property is only available on Windows and Linux platforms.
For cross-platform compatibility, use ＃If...＃Endif with the Target... specifiers to make sure you will not attempt to use this property on an incompatible platform.
The Alt key on Windows and Linux keyboards. If selected, the Alt key is must be held down while pressing the value of the Key property in order to trigger the event handler for the MenuItem. This property is for Windows and Linux only.
Use MenuItem.Shortcut to change this value at runtime.