Hi lancaster-mjf
You will have to register for MTD at HMRC, once you have received the confirmation email from HMRC you will then have to set up MTD from within the product.
Select the cog/gear icon > account and settings > advanced > Making Tax Digital > Learn how to connect.
Navigate to the Taxes section and to the right of the prepare return option of the open period use the drop down to select submit with spreadsheet.
The above option is not available if you are using the Flat Rate Scheme.