Hi Zain,
You have introduced AppName function in OSMDb_Core_TestApp module. So Use AppName() function to show name of app in screen.
Your expression would be like below.
"Hello from the " + AppName() + " app."
Note: You have to use your function AppName() in OSMDb_Test module. Because you just introduce this function in OSMDb_Core.

Regards,
Rahul