Implement BOOT_COMPLETED receiver.  NOTE:  You must manually add the android permission android.permission.RECEIVE_BOOT_COMPLETED to your AndroidManifest -- the plugin has this element in its plugin.xml purposely commented-out.  Also, when the BackgroundGeolocationService is run due to a BOOT_COMPLETED, there's no way to fetch any kind of user info, such as an authentication_token for your server to authenticate an HTTP request (if you've enabled the HTTP feature).  However, the HTTP request will insert the Android UUID which you *can* use to authenticate the request.  It's up to the developer to map the Android UUID to some particular user on your server.  You may fetch the Android UUID using the standard cordova plugin org.apache.cordova.device (see the documentation there on that API)