Вставляем в public OnPlayerCommandText.

Код:
else if(strcmp(cmd, "/msg", true) == 0 || strcmp(cmd, "/message", true) == 0)
    {
     if (PlayerInfo[playerid][pAdmin] >= 4)
      {
        if(IsPlayerConnected(playerid))
        {
            new sendername[MAX_PLAYER_NAME];
            new string[124];
            GetPlayerName(playerid, sendername, sizeof(sendername));
            new length = strlen(cmdtext);
            while ((idx < length) && (cmdtext[idx] <= ' '))
            {
                idx++;
            }
            new offset = idx;
            new result[64];
            while ((idx < length) && ((idx - offset) < (sizeof(result) - 1)))
            {
                result[idx - offset] = cmdtext[idx];
                idx++;
            }
            result[idx - offset] = EOS;
            if(!strlen(result))
            {
                SendClientMessage(playerid, -1, "Используйте: /msg [text]");
                return 1;
            }
            format(string, sizeof(string), "{FFCD00}Администратор %s: %s", sendername, result);
            SendClientMessageToAll(0xFFCD00AA, string);
        }
        return 1;
    }
        }

http://images.vfl.ru/ii/1377812739/0e979545/2998747.png