var wnd: HWND;
wnd = FindWindow('Shell_TrayWnd', 0);    
if (wnd <> 0)  then
  SendMessage(wnd, WM_COMMAND, 0x019f, 0);  //Свернуть все окна Windows

unit TestHookMessages;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, Vcl.ExtCtrls,  System.TypInfo;

type
  TForm1 = class(TForm)
    Panel1: TPanel;
    Memo1: TMemo;
    procedure WM_Size(var Msg:tagMSG); message WM_SIZE;
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.WM_Size(var Msg: tagMSG);
begin
  inherited;
  case Form1.WindowState of
    wsNormal, wsMaximized:
      Memo1.Lines.Add('Окно приложения на рабочем столе');
    else
      Memo1.Lines.Add('Окно приложения свернуто!');
  end;
end;

end.