Brave Desktop:

1. Go to Settings → Privacy and security (brave://settings/privacy).
2. Enable "Use Google services for push messaging" and then click Relaunch.
3. Open your key in nsec.app and click on Enable background service badge.
4. You're all set!

Brave Android:

There is no need for any special settings for Brave on Android.

Brave iOS:

1. Go to iOS Settings → Advanced → Experimental.
2. Enable Push API
3. Open use.nsec.app in Brave.
4. Choose Add to homescreen option to install Nsec.app.
5. You're all set!