Send String from One client to Another using UDP

As what i'm thinking you're trying to make simple chat system, maybe. And this is Unity Answer so i suggest a solution in Unity Networking only.

1.Create GameObject call Server, attached Server.cs with script below :

 using UnityEngine;
 using UnityEngine.Networking;
 using System.Collections;
 
 public class Server : MonoBehaviour
 {
     void Start () 
     {
     listenPort = 8888;
         //Listening...
         NetworkServer.Listen(listenPort);
 
         //System handle
         NetworkServer.RegisterHandler(MsgType.Connect, OnClientConnected);
         NetworkServer.RegisterHandler(MsgType.Disconnect, OnClientDisconnected);
         
         //Custom handle - here is your string come
         NetworkServer.RegisterHandler(ZMessageType.String, OnServerReceiveStringMessage);
      }
     
      //Receive function
      void OnServerReceiveStringMessage(NetworkMessage netMsg)
      {
         var stringMsg = netMsg.ReadMessage<StringMessage>();
          Debug.Log("You string text: " + stringMsg.stringText)
         //Now you can do anything here to display on screen in the first "open copy"
      } 
 
     //Sending function
     public void OnServerSendStringMessage(int connectionId)
    {
        StringMessage stringMsg = new StringMessage();
        stringMsg.stringText = "Your message here, write what u wanna do";
        NetworkServer.SendToClient(connectionId, ZMessageType.String, stringMsg);
    }
 }

2.Create GameObject called Client and attach Client.cs below :

 using UnityEngine;
 using UnityEngine.Networking;
 using System.Collections;
 
 public class Client : MonoBehaviour 
 {
     public NetworkClient m_Client = new NetworkClient();
     void Start () 
     {
         string serverIP = 127.0.0.1; //Local is 127.0.0.1, if u test online server, please change.
         int serverPort = 8888;    //same as Server.cs port
 
         //Connecting...
         m_Client.Connect(serverIP, serverPort);
 
         //System handle
         m_Client.RegisterHandler(MsgType.Connect, ClientConnected);
         m_Client.RegisterHandler(MsgType.Disconnect, ClientDisconnected);
 
         //Custom handle, u message listen here
         m_Client.RegisterHandler(ZMessageType.STRING,ClientReceiveStringMessage);
     }
 
     //Receive function
     void ClientReceiveStringMessage(NetworkMessage netMsg)
     {
         var stringMsg = netMsg.Read<StringMessage>();
         Debug.Log("Your message is " + stringMsg.stringText);
         //Do what u wanna do here, display UI, etc..
     }
     //Send function
     //Same like Server.cs, just copy
 }

3.Create a C# class have any name u want, i create ZMessage.cs, don't attach to any object please

     using UnityEngine;
     using UnityEngine.Networking;
     using System.Collections;
    
     public class ZMessageType
     {
         public static short STRING          = 5000;
     }
     
     public class StringMessage : MessageBase
     {
         public string stringValue;
     }