Delphi: Define FormStyle fsMDIChild or fsNormal at runtime

I suggest to use TFrame container. It is like TForm but without border. So, add TFrame to project. Add all components and code to that unit like you do on your form. Then you can create TForm on runtime as TMdiChild or Dialog. This is the way to put your frame control to other form....

... when you want to create dialog:

procedure TForm1.Button1Click(Sender: TObject);
var
  frm: TFrame1;
  modalFrm: TForm;
begin
  modalFrm := TForm.Create(Self);
  try
    modalFrm.BorderStyle := bsDialog;
    modalFrm.Position := poScreenCenter;
    modalFrm.Width := 500;
    modalFrm.Height := 400;

    frm := TFrame1.CreateParented(modalFrm.Handle);
    frm.Parent := modalFrm;
    frm.Align := alClient;
    frm.Visible := True;

    modalFrm.ShowModal;
  finally
    modalFrm.Free;
  end;
end;

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... when you want to put frame on form as mdichild:

var
  frm: TFrame1;
  mdiFrm: TForm;
begin
  mdiFrm := TForm.Create(nil);
  try
    mdiFrm.FormStyle := fsMDIChild;
    mdiFrm.Parent := Self;
    mdiFrm.Width := 500;
    mdiFrm.Height := 400;

    frm := TFrame1.CreateParented(mdiFrm.Handle);
    frm.Parent := mdiFrm;
    frm.Align := alClient;
    frm.Visible := True;

    mdiFrm.Show;
   // mdiFrm.WindowState := wsMaximized;
  finally
  end;

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... when you put frame on main form which is not mdiparent but simple form
(can be your TPanel component if want):

var
  frm: TFrame1;
begin
  frm := TFrame1.CreateParented(Self.Handle); // or Panel1.Handle
  frm.Parent := Self;  // or Panel1
  frm.Align := alClient;
  frm.Visible := True;

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