To access the ntfy CLI, simply open a Web terminal and run ntfy:

root@89c61cad-e48d-4fce-bcc2-b82b3b1b8a69:/app/code# ntfy
NAME:
   ntfy - Simple pub-sub notification service

USAGE:
   ntfy [OPTION..]

COMMANDS:
...

CLI configuration can be adjusted in /app/data/config/client.yml.