unit Unit5;

interface

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

const
  WM_MYHANDLER = WM_USER + 1;
type
  TForm5 = class(TForm)
    procedure FormCreate(Sender: TObject);
  private
    procedure WMMyHandler(var Message: TMessage); message WM_MYHANDLER;
  end;

var
  Form5: TForm5;

implementation

{$R *.dfm}

procedure TForm5.FormCreate(Sender: TObject);
begin
  PostMessage(Handle, WM_MYHANDLER, 0, 0)
end;

procedure TForm5.WMMyHandler(var Message: TMessage);
begin
  ShowMessage('Выполняю некоторый код')
end;

end.