You need to override the CreateParam function and there add WS_EX_TRANSPARENT
to the Params.ExStyle.
Set the form's canvas' Brush.Style to bsClear, as shown in this example:
type
TMyForm = class(TForm)
procedure FormCreate(Sender: TObject);
procedure CreateParams(var Params: TCreateParams); override;
end;
procedure TMyForm.CreateParams(var Params: TCreateParams);
begin
inherited CreateParams(Params);
// this is the important constant!
Params.ExStyle:= Params.ExStyle or WS_EX_TRANSPARENT;
end;
Procedure TMyForm.FormCreate(Sender: TObject);
begin
inherited;
Canvas.Brush.Style := bsClear;
end;