Meta is rolling out the ability to link your account to Discord to share what Quest app or Horizon World you're currently in.
At Connect 2025, Meta announced that Discord was coming to Quest in 2026, including the ability to share your status to show friends what you're playing.
The Discord app for Quest is still set for 2026, so isn't here yet. But what is now "rolling out", seemingly ahead of schedule, is the status sharing feature.

You can set it up on the web in the App Connections section of the Meta Accounts Center on the web, or in the mobile app at Accounts Center --> Your information and permissions --> App connections.
If you don't see Discord listed yet, it means it hasn't rolled out to your account yet, so you need to check again at a later date.
Meta says that the rollout is "gradual" in case there are any issues or bugs.

Note that for the app or world you're in to show up you'll need to have your Horizon Active Status set to Online or Joinable, and you can thus hide your current activity by switching to Offline.