Because the Alt key is a reserved Windows key for accessing menu items etc. you have to capture it a bit differently than the other modifier keys like Shift or Ctrl.

I found an example on MSDN and converted it to COBOL:

    working-storage section.
    01 AltKeyPressed condition-value value false.
    method-id Window_PreviewKeyDown.
       procedure division using by value sender as object e as type System.Windows.Input.KeyEventArgs.
              
          if AltKeyPressed
             set AltKeyPressed to false
             if (e::SystemKey = type Key::U)
                set mytextBox::Text to "Alt U is pressed!"
             else
                set mytextBox::Text to " "
             end-if
          end-if

          if (e::SystemKey = type Key::LeftAlt)
             set AltKeyPressed to true
          end-if.
      
       end method.