I’m thinking there is some name confusion. The service used is Discourse. The Discord app (mainly used by gamers) doesn’t, so far as I know doesn’t have an interface to Discourse.
This isn’t something you as the user can control; iOS doesn’t provide any facility for users to point specific HTTP URLs to particular apps, so they’ll be handled by Safari or an internal WebKit browser in your email app.
The Discourse app developers could change their app to work this way. Here’s some information on how that works:
And then someone would need to suggest this feature in the Discourse support forum at
I’m not entirely certain if there’s an advantage to using the Discourse app over Safari; I haven’t noticed much difference since the app basically shows the Web site.
You can at least control it on one side of the equation, though. If you don’t like the app and don’t want the links opening in the app, delete the app.