Main.java (Plugin Sponge)
    @Listener
    public void onInitialization(GameInitializationEvent event){
        messageChannel = Sponge.getGame().getChannelRegistrar().getOrCreateRaw(this, "rpc");
        messageChannel.addListener(new ModListener());
    }

ModListener.class (Plugin Sponge)
    @Override
    public void handlePayload(ChannelBuf data, RemoteConnection connection, Platform.Type side) {
        if (!(connection instanceof PlayerConnection)) {
            System.out.println("not a player");
            return;
        }

        PlayerConnection playerConnection = (PlayerConnection) connection;
        User user = CacheUtil.getUser(playerConnection.getPlayer().getUniqueId());

        playerConnection.getPlayer().sendMessage(Text.of("received"));

        System.out.println("received");
        System.out.println(data.readUTF());
    }

Main.java (Mod Forge)
    @Mod.EventHandler
    public void init(FMLServerStartingEvent event) {
        MinecraftForge.EVENT_BUS.register(new ListenerClass());
    }

ListenerClass.java (Mod Forge)
    @SubscribeEvent
    public void onPlayerLogin(InputEvent.KeyInputEvent event) {
        ByteBuf buffer = Unpooled.buffer();
        ByteBufUtils.writeUTF8String(buffer, "test");
        Minecraft.getMinecraft().player.connection.sendPacket(new CPacketCustomPayload("rpc", new PacketBuffer(buffer)));
    }

    @SubscribeEvent
    public void onClientPacket(FMLNetworkEvent.ClientCustomPacketEvent event) {
        String data = ByteBufUtils.readUTF8String(event.getPacket().payload());
        Minecraft.getMinecraft().player.sendChatMessage(data);
    }