I have no idea how Torbox add works since I only have the basic version and only use the regular debrid with Torbox and use Easynews for Usenet. I think they are using another service and just passing their search through. I don’t know how their streaming of Usenet works.
To use the the Easynews sub you would subscribe to Newshosting, go to your dashboard and activate your Easynews sub and then go to set up your Easynews addon and input your Easynews credentials.
https://stremio-addons.com/?q=easynews
From there it will act like any other Stremio addon.