ReRealms

API

Get Realm ID from Bukkit World

World bukkitWorld = player.getWorld();//...
String worldId = bukkitWorld.getMetadata("realm_id").getFirst().asString();

or a more useful approach

PaperRealmsAPI api = RealmsAPIProvider.getPlatformAPI(PaperRealmsAPI.class);
LocalLoadedRealmWorld realmWorld = api.getLocalLoadedRealmWorld(bukkitWorld);