init.sqf - Init script on client side

player addAction ["Send Message",
{
  _clientId = clientOwner;
  _name = name player;
  toTheServer = [_clientId, _name];
  publicVariableServer "toTheServer"
}];

"toTheClient" addPublicVariableEventHandler
{
  private ["_data"];
  _data = _this select 1;
  _message = _data select 0;
  _name  = _data select 1;

  _newMessage = _message + _name;
  hint _newMessage;
};

initServer.sqf - Init script on server side

"toTheServer" addPublicVariableEventHandler
{
  private ["_data"];
  _data = (_this select 1);
  _clientId = (_this select 0);
  _playerName = (_this select 1);

  _message = "Joined the server"
  toTheClient = [_message, _playerName];
  _clientId publicVariableClient "toTheClient";

};