To handle the case where a user changes notification settings from outside the app (e.g., enabling notifications from device settings)

Ionic Framework
1

to handle the case where a user changes notification settings from outside the app (e.g., enabling notifications from device settings).
then how to generate token and how to call functions to check token is there or not , if not how will create and after create how to send token to API without any user Interface.

Or any other possibility to handle this case ?