Hi,
If you’re on Mac, you can make a shortcut for it pretty easily. Just follow the basic directions here for adding a shortcut, but instead of making the shortcut for “Negative Lab Pro” make it for “Flip Horizontal” and set it to whatever shortcut you’d like.
If you on Windows, there isn’t a direct way to create a shortcut for it.
BUT, in either case, please note that in Lightroom you can Flip Horizontal on many photos at once. Just make sure you are in the Library (hit “g” on your keyboard), and select all the photos you want to flip, then go to “Photo > Flip Horizontal” (or use your keyboard shortcut if you set one up on mac), and it will do this to all the images you have selected.
This is easy to do in Lightroom itself, so I don’t imagine I will ever try to duplicate this feature in Negative Lab Pro.
-Nate