If your question is only related to the way to put the form in a TPanel then it's quite easy.
You only need to call Windows.SetParent() with the handle of the newly created form.
This would also work by using the TForm Property ParentWindow.
But it's (quite) possible that the transparency is not handled (I briefly tested with a semi opaque form and it's not drawn correctly, So if the transparency is mandatory it mights be impossible to reach your goal...)
If your question is only related to the way to put the form in a TPanel then it's quite easy. You only need to call Windows.SetParent() with the handle of the newly created form.
This would also work by using the TForm Property ParentWindow. But it's (quite) possible that the transparency is not handled (I briefly tested with a semi opaque form and it's not drawn correctly, So if the transparency is mandatory it mights be impossible to reach your goal...)