Registering on Client (FORGE):
network = NetworkRegistry.INSTANCE.newSimpleChannel("DiscRichPresenceMod_");
network.registerMessage(PluginMessage.Handler.class, PluginMessage.class, 0, Side.CLIENT);
Registering on Server (BUKKIT/SPIGOT):
Bukkit.getMessenger().registerOutgoingPluginChannel(this, "DiscRichPresenceMod_");
Bukkit.getMessenger().registerIncomingPluginChannel(this, "DiscRichPresenceMod_", new DiscordPluginMessageManager(this));