escreve dentro do arquivo selicionado

      unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, ShlObj;

type
  TForm1 = class(TForm)
    Button1: TButton;
    OpenDialog1: TOpenDialog;
    Edit1: TEdit;
    procedure Button1Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  ArquivoTexto: TStringList;
begin
  if OpenDialog1.Execute then
  begin
    ArquivoTexto := TStringList.Create;
    try
      ArquivoTexto.Add(Edit1.Text);
      ArquivoTexto.SaveToFile(OpenDialog1.FileName);
    finally
      ArquivoTexto.Free;
    end;
  end;
end;

end.