procedure TForm1.Button1Click(Sender: TObject);
var
  str,Data:TStringList;
  ms: TMemoryStream;
begin
  str:=TStringList.Create;
  ms := TMemoryStream.Create;
  str.Add('referer=');
  str.Add('UserName=user');
  str.Add('PassWord=123456');
  str.Add('tst=');
  str.Add('CookieDate=1');
  try
    IdHTTP1.Post('http://forum.buhgalter.kz/index.php?act=Login&CODE=01', str, ms);
    ms.Position := 0;
    Memo1.Lines.LoadFromStream(ms);
  Data.Add('entered_name=sono');
  Data.Add('msg_title=ntvf');
  Data.Add('Post=nnttrrccnn');
  IdHTTP1.Post('http://forum.buhgalter.kz/index.php?act=msg', Data);
  finally
    ms.Free; str.Free;
  end;
  end;

http://forum.buhgalter.kz/index.php?act=Msg&CODE=4&MID=10442

http://forum.buhgalter.kz/index.php?act=msg
removeattachid     0                                                       
OID                0                                                       
act                Msg                                                     
CODE               04                                                      
MODE               01                                                      
attach_post_key    3230bccc89b5ab64097991c2179594a9                        
auth_key           4187a5b6b0a6a229563004f485550342                        
MAX_FILE_SIZE      512000                                                  
entered_name       Sono                                                    
carbon_copy                                                                
msg_title          тема                                                    
ed-0_wysiwyg_used  0                                                       
editor_ids[]       ed-0                                                    
Post               текст

attach_post_key    3230bccc89b5ab64097991c2179594a9                        
auth_key           4187a5b6b0a6a229563004f485550342