Q:  What is the easiest way to change the control menu of a form based application ?

Example, I was to add an option called "Always on top" that can be checked and unchecked at the user's will.  This of course, should be there in addition to the "Move", "Restore", "Maximize" etc. other control menu options.

A:  type
  TForm1 = class(TForm);
  {...}
  private
    procedure WMSysCommand(VAR Message: TWMSysCommand);
    message WM_SYSCOMMAND;
...

procedure TForm1.WMSysCommand(var Message: TWMSysCommand);
begin
  Inherited;
  IF Mesage.CmdType AND $FFF0 = $F200 THEN
    MessageBeep(0); {replace with code to DO something}
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  AppendMenu(GetSystemMenu(Handle, False), MF_STRING, $F200, '&Wahoo!');
end;

       This adds a new item named "Wahoo!" to the system menu, with a command ID of $F200. I picked $F200 at random as a number near the regular SC_xxxx constants, but greater than any of them. Note that the ID for a
system command must be evenly divisibly by 16, as Windows uses the lowest 4 bits of the ID. That's also why you AND the value with $FFF0 before comparing it to the specific ID.
 

Here is a template for changing the system menu:
 

unit Unit1;

interface

uses
  SysUtils, WinTypes, WinProcs, Messages, Classes, Graphics, Controls,
  Forms, Dialogs, Menus;

type
  TForm1 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    { Private declarations }
  public
     procedure winmsg(var msg:tmsg;var handled:boolean);
     {This is what handles the messages}

     procedure DOWHATEVER;{procedure to do whatever}
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}
const ItemID=99;{the ID number for your menu item--can be anything}

procedure tform1.winmsg(var msg:tmsg;var handled:boolean);
begin
  if msg.message=wm_syscommand then{if the message is a system one...}
   if msg.wparam = ItemID then DOWHATEVER;{then check if its parameter
                                           is your Menu items ID,}
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  application.onmessage:=winmsg;
  {tell your app that 'winmsg' is the application message handler}

  AppendMenu(GetSystemMenu(form1.handle,false),mf_separator,0,'');
  {Add a seperator bar to form1}
 
AppendMenu(GetSystemMenu(form1.handle,false),mf_byposition,ItemID,
  '&New Item');
{add your menu item to form1}

 
AppendMenu(GetSystemMenu(application.handle,false),mf_separator,0,'');
{Add a seperator bar to the application system menu(used when app
 is minimized)}
 
AppendMenu(GetSystemMenu(application.handle,false),mf_byposition,
ItemID,'&New Item'
{add your menu itemto the application system menu(used when app is
 minimized)}

{for more information on the AppendMenu and GetSystemMenu see online
 help}

end;

procdure TForm2.DOWHATEVER;
begin
 {add whatever you want to this procedure}
end;

end.