UDPSender.cs

using System.Net.Sockets;
using System.Text;
using UnityEngine;

public class UDPSender : MonoBehaviour
{
    private UdpClient client;

    void Start()
    {
        client = new UdpClient();
        client.Connect("localhost", 3333);
    }

    void OnGUI()
    {
        if (GUI.Button(new Rect(10, 10, 100, 40), "Send"))
        {
            byte[] dgram = Encoding.UTF8.GetBytes("hello!");
            client.Send(dgram, dgram.Length);
        }
    }

    void OnApplicationQuit()
    {
        client.Close();
    }
}