I did a search on the NVidia site looking for infrared and found a support forum.[Link: nvidia.com]
Found several links in the thread I found.
Scroll down about halfway down and you will see a post that has this + several links
OK / Select: 61453
Mute / Unmute: Nothing
Stop: 61628[Link: support-shield.nvidia.com][Link: forum.kodi.tv]
Looks like the actual Pronto Hex got removed from the forum but there's a handy-dandy link I found which also includes Power On/Off that you can test.
Here's the link you are looking for.[Link: pastebin.com]
I'm also curious as to why you did not create a new thread for this request vs using a Firestick thread. Are they really the same thing?