Just create a shortcut in your desktop and assign a Hotkey:
For example: This is for People app
1. Create a new shortcut in your desktop: using the following format
%windir%\explorer.exe [App's URL Protocol name]: <~ ending with colon
App's URL Protocol name? read about it at the end
2. Open its properties dialog and setup the hotkeys you want to use
And that's it, now you can launch your Windows 8 applications with that hotkey
App's URL Protocol
Some applications have their own URL protocol to open its supported files, something similar to http:// or mailto:
And you will find those names in windows registry : HKEY_CURRENT_USER\Software\Classes
There you will find some of your applications (not all of them), just look for the ones with default value like this one:
URL:wlpeople
Another hint is that they also have a string value called URL Protocol
Hopefully, you will find your desired-application's URL Protocol name. Good luck!