You would do the following to assign MIDI events to navigation zones or hot corners:
Open OnSong--> Utilities Menu--> Editors--> Navigation--> select the Screen tab from the bottom--> Tap on the navigation area you would like to map--> scroll down and select Send MIDI--> Tap the + button to add a MIDI event--> Configure your MIDI event to suit your needs--> Tap Done--> Tap Back--> Tap onto the Customization navigation box--> Tap Close.