public void handleLogin(Packet1Login par1Packet1Login)
    {
  Packet250CustomPayload plugin = new Packet250CustomPayload();
        plugin.channel = "REGISTER";
        plugin.data = "CustomGui".getBytes();
        this.mc.getNetHandler().addToSendQueue(plugin);
}


----------------------------------------------------------------------------------

package com.topicarp.spenk.customgui;

import net.canarymod.Canary;
import net.canarymod.Logman;
import net.canarymod.commandsys.CommandDependencyException;
import net.canarymod.plugin.Plugin;

public class CustomGui extends Plugin{

    @Override
    public void disable() {
        Logman.println("CustomGui disabled");

    }

    @Override
    public boolean enable() {
        try {
            Canary.hooks().registerListener(new CanaryGuiListener(), this);
            Canary.commands().registerCommands(new CustomGuiCommand(), this, false);
        } catch (CommandDependencyException e) {
            e.printStackTrace();
        }
        Logman.println("CustomGui enabled");
        return false;
    }

}


-----------------------------------------------------------------------------------


package com.topicarp.spenk.customgui;

import net.canarymod.Canary;
import net.canarymod.api.entity.living.humanoid.Player;
import net.canarymod.chat.MessageReceiver;
import net.canarymod.commandsys.Command;
import net.canarymod.commandsys.CommandListener;

public class CustomGuiCommand implements CommandListener{
    @Command(aliases = { "gui" }, description = "gui",permissions = { }, toolTip = "/gui", min = 1)
    public void GuiCommand(MessageReceiver caller, String[] parameters) {
        caller.message("§cPackage preparing sent");
        if (caller instanceof Player){
            Player player = (Player)caller;
            player.sendMessage("§cPackage almost sent");
        byte[] command = "OpenGui".getBytes();
        Canary.channels().sendCustomPayloadToPlayer("CustomGui", command, player);
        player.sendMessage("§cPackage sent");
        }
    }
}


--------------------------------------------------------------------------------------

package com.topicarp.spenk.customgui;

import net.canarymod.Canary;
import net.canarymod.hook.player.ConnectionHook;
import net.canarymod.plugin.PluginListener;

public class CanaryGuiListener implements PluginListener{

    public void loginHook(ConnectionHook hook){
        Canary.channels().registerClient("CustomGui", hook.getPlayer().getNetServerHandler());
    }

}