source: TFP-WebServer/CommandExtensions/src/ChatHelpers.cs@ 458

namespace CommandExtensions {
        public static class ChatHelpers {
                public static void SendMessage (ClientInfo _receiver, ClientInfo _sender, string _message) {
                        string senderName;
                        if (_sender != null) {
                                PrivateMessageConnections.SetLastPMSender (_sender, _receiver);
                                senderName = _sender.playerName;
                        } else {
                                senderName = "Server";
                        }

                        _receiver.SendPackage (NetPackageManager.GetPackage<NetPackageChat> ().Setup (EChatType.Whisper, -1, _message,
                                $"{senderName} (PM)", false, null));
                        string receiverName = _receiver.playerName;
                        receiverName = receiverName != null ? $"\"{receiverName}\"" : "unknownName";
                        SdtdConsole.Instance.Output ($"Message to player {receiverName} sent with sender \"{senderName}\"");
                }
        }
}