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type
  TForm2 = class(TForm)
  ...
  public
    Test: String;
  end;

// ODER:
  TForm2 = class(TForm)
  ...
  private
    FTest: String;
  public
    property Test: String read FTest write FTest;
  end;

...

procedure TForm2.Button1Click(Sender: TObject);
begin
  Test := 'Hallo!';
end;					

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Form2.Test := 'Test';
Form2.Show;
ShowMessage(Form2.Test);