Unfortunately, you can't change the target folder from the internal storage to the external micro SD. The application is configured to transfer files only to internal storage. For further information I suggest you contact the support team using form from the Help menu:
If you want, you can move the files to SD card directly on your Android device. Open the File Explorer/My Files utility on your phone, tap and hold your finger on the files/folder you want to transfer, select Move and follow the on-screen instructions.