But to launch a custom app you might want to install a 'keyboard shortcut' app from the Play Store, connect up a key combination, and then make Puck.js output that key combination
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
To be able to completely control what's going on you'll probably need to do your own app.
However a simple solution is to set up your Puck.js to pretend to be a keyboard: http://www.espruino.com/BLE+Keyboard
If you want a 'standard' app like the music player then just pressing the 'play' button might be enough - there are some examples of shortcuts here: https://www.howtogeek.com/175267/the-htg-guide-to-using-a-bluetooth-keyboard-with-your-android-device/
But to launch a custom app you might want to install a 'keyboard shortcut' app from the Play Store, connect up a key combination, and then make Puck.js output that key combination