procedure ToolTip_Create(hWnd: HWND; const lpText: PChar);
var
 TI: TToolInfo;
 hTip: Windows.HWND;
begin
 hTip := CreateWindowEx(WS_EX_TOPMOST, TOOLTIPS_CLASS, nil,
   WS_POPUP or TTS_NOPREFIX or TTS_ALWAYSTIP,
   LongInt(CW_USEDEFAULT),
   LongInt(CW_USEDEFAULT),
   LongInt(CW_USEDEFAULT),
   LongInt(CW_USEDEFAULT),
   hWnd, 0, hInstance, nil);

 FillChar(TI, SizeOf(TToolInfo), 0);

 TI.cbSize := SizeOf(TToolInfo);
 TI.uFlags := TTF_IDISHWND or TTF_SUBCLASS or TTS_NOPREFIX or TTS_ALWAYSTIP;
 TI.hwnd := hWnd;
 TI.uId := hWnd;
 TI.hInst := hInstance;
 TI.lpszText := lpText;

 SendMessage(hTip, TTM_ADDTOOL, 0, LPARAM(@TI));
end;

procedure TForm1.FormCreate(Sender: TObject);
var
 H: HWND;
begin
 H := CreateWindow('Button', 'Кнопка', WS_CHILD or WS_VISIBLE or BS_PUSHBUTTON,  300, 300, 100, 50, Handle, 0, 0, nil);

 ToolTip_Create(H, 'Опа');
end;