if myreg.OpenKey('*\shell\MyApp\command',True)
then begin
  myreg.WriteString('','"'+TntApplication.ExeName+'" "%1"');
  myreg.CloseKey;
end;

procedure TTntRegistry.WriteString(const Name, Value: WideString);
begin
   PutData(Name, PWideChar(Value), (Length(Value)+1) * SizeOf(WideChar), rdString);
end;

procedure TTntRegistry.PutData(const Name: WideString; Buffer: Pointer;
  BufSize: Integer; RegData: TRegDataType);
var
  DataType: Integer;
begin
    DataType := RegDataToDataType(RegData);
    if RegSetValueExW(CurrentKey, PWideChar(Name), 0, DataType, Buffer,
      BufSize) <> ERROR_SUCCESS then
      raise ERegistryException.CreateResFmt(@SRegSetDataFailed, [Name]);
end;