Start a new project.

Place a Panel on Form1, resize it so it is reasonably large.

Add a new Form (Form2).

Add this code to Form2.

procedure TForm2.FormCreate(Sender: TObject);
begin
    Form2.Parent := Form1.Panel1;
    Form2.Visible := True;
    Form2.WindowState := wsMaximized;
end;

If you change the color of Form2 it looks nicer.

Run the program.

You should now see Form1 with a panel on it and Form2 inside the panel.

You can minimize Form2 and then maximixe it.