در قسمت Wnd . باید Handle فرم را بدهید.مثلا MakeWndTrans(Form1.Handle,50)?

function MakeWndTrans(Wnd: HWND; nAlpha: Integer = 10): Boolean;
type
  TSetLayeredWindowAttributes = function(hwnd: HWND; crKey: COLORREF; bAlpha: Byte;
    dwFlags: Longint): Longint; stdcall;
const
  // Use crKey as the transparency color.
  LWA_COLORKEY = 1;
  // Use bAlpha to determine the opacity of the layered window..
  LWA_ALPHA = 2;
  WS_EX_LAYERED = $80000;
var
  hUser32: HMODULE;
  SetLayeredWindowAttributes: TSetLayeredWindowAttributes;
  i : Integer;
begin
  Result := False;
  // Here we import the function from USER32.DLL
  hUser32 := GetModuleHandle("USER32.DLL");
  if hUser32 <> 0 then
  begin
    @SetLayeredWindowAttributes := GetProcAddress(hUser32,"SetLayeredWindowAttributes");
    // If the import did not succeed, make sure your app can handle it!
    if @SetLayeredWindowAttributes <> nil then
    begin
      // Check the current state of the dialog, and then add the WS_EX_LAYERED attribute
      SetWindowLong(Wnd, GWL_EXSTYLE, GetWindowLong(Wnd, GWL_EXSTYLE) or WS_EX_LAYERED);
      // The SetLayeredWindowAttributes function sets the opacity and
      // transparency color key of a layered window
      SetLayeredWindowAttributes(Wnd, 0, Trunc((255 / 100) * (100 - nAlpha)), LWA_ALPHA);
      Result := True;
    end;
  end;
end;