RDSSpy play stream reads RDS data (*) from TCP host:port. Default host is localhost and port is 3122.
* https://teknokoodiradio.vuodatus.net/lue/2019/11/sdr-rds-groups-to-rdsspy-spy-format-file-plugin

Set RDS source to (No Source)

rds1.spy file content:
620356A6138554F
62031560E1CC7
6203E56145206207
620356F7C614D49
6203256D20202020
.....

using System;
using System.IO;
using System.Net;
using System.Net.Sockets;
using System.Text;

namespace TCPlistener
{
    class Program
    {
        static void Main(string[] args)
        {
            TcpListener server = null;
            try
            {
                Int32 port = 3122;
                IPAddress localAddr = IPAddress.Parse("127.0.0.1");
                server = new TcpListener(localAddr, port);
                server.Start();
                NetworkStream stream = null;
                TcpClient client = null;
                while (true)
                {
                    Console.Write("Waiting for a connection... ");
                    client = server.AcceptTcpClient();
                    stream = client.GetStream();
                    Console.WriteLine("Connected!");
                    break;
                }
                using (StreamReader sr = new StreamReader("c:\\utils\\sdr\\rds1.spy"))
                {
                    string line;
                    while ((line = sr.ReadLine()) != null)
                    {
                        line = line + Environment.NewLine;
                        byte[] rds = Encoding.ASCII.GetBytes(line);
                        stream.Write(rds, 0, rds.Length);

// some delay is needed otherwise RDSSpy produces errors
                        System.Threading.Thread.Sleep(50);
                        Console.WriteLine(line);
                    }
                }
                client.Close();
            }
            catch (SocketException e)
            {
                Console.WriteLine("SocketException: {0}", e);
            }
            finally
            {
                server.Stop();
            }

            Console.WriteLine("\nHit enter to continue...");
            Console.Read();
        }
    }
}