This is simple example, that shows how to create auto reply bot in Minecraft Forge 1.18.2

AutoReply.java

import net.minecraftforge.client.event.ClientChatReceivedEvent;
import net.minecraftforge.event.entity.player.PlayerInteractEvent;
import net.minecraftforge.event.entity.player.AttackEntityEvent;
import net.minecraftforge.eventbus.api.EventPriority;
import net.minecraftforge.eventbus.api.SubscribeEvent;
import net.minecraftforge.fml.common.Mod;
import net.minecraftforge.api.distmarker.Dist;
import net.minecraft.client.Minecraft;
import net.minecraft.client.player.LocalPlayer;

@Mod.EventBusSubscriber(modid = SkytopAutomizer.MOD_ID, bus = Mod.EventBusSubscriber.Bus.FORGE, value = Dist.CLIENT)
public class FastMath {
  
   @SubscribeEvent
   public static void onClientChat(ClientChatReceivedEvent event) {
       String keyword = "msg";
       // In newest version you need to use getString() method
       if (event.getMessage().getString().contains(keyword)) {
           Minecraft.getInstance().player.chat("/reply hi");
           // You need to use chat method, not sendMessage (or addMessage)
       }
   }
}