Simplest option would be that the Plex app is able to listen to intents, I’m that case Tasker won’t have to add anything - but also won’t have a nice UI for the integration, instead you have to fill in the commands yourself. Otherwise I guess Tasker needs to add something too I guess…
As I understood it when using it with Poweramp, Tasker just sends some sort of cli command to an open interface on the app. so tasker won’t have to update anything on their end. Intents just trigger activities in your app