You cannot run shell commands without the shell itself(cmd.exe)!
Running stand-alone apps, like Notepad, etc, don't require a shell, so they can be run directly.
To run a shell command in Windows, use the following code:
For example, to run the referenced "copy" command, use this:
cmd /C copy "!" "!.backup"
Hope this helps someone else ;)
To WinSCP team
: Executing local OS shell commands isn't covered in your official documentation(at least from my searches of it). Please consider adding this important topic to your Custom Command documentation. Thanks!