If you look up this version of the Get function in FileMaker help, you'll find that there is no windows equivalent key for this value. You'll need to detect the platform and test for a different key, such as Alt ( value is 8 ) for windows systems.
How would I wright the script:
Get ( ActiveModifierKeys )=16 //for mac
Get ( ActiveModifierKeys )=8 // for windows?
Check your get functions in FileMaker Help. I forget the exact parameter used, but there's one that will tell you whether the current platform is Mac or Windows.