//Set your form to Ellipse region
procedure TForm1.Button1Click(Sender: TObject);
var
hRGN: THandle;
begin
hRGN := CreateEllipticRgn(0,0,width,height);
SetWindowRgn(Handle, hRgn, true);
end;
//Set your form to RoundRect region
procedure TForm1.Button2Click(Sender: TObject);
var
hRGN: THandle;
begin
hRGN := CreateRoundRectRgn(0,0,width,height,Width div 4, Height div 4);
SetWindowRgn(Handle, hRgn, true);
end;
//Set button1 to Ellipse region
procedure TForm1.Button3Click(Sender: TObject);
var
hRGN: THandle;
begin
hRGN := CreateEllipticRgn(0,0,Button1.width,Button1.height);
SetWindowRgn(Button1.Handle, hRgn, true);
end;