Start off by listing your property with a "for sale" status in Base> list it and close it.
Now Open the relevant listing again
Click on the "Diary" item on the right-hand side of the mandate screen
Select the correct date on the calendar and click on "Add"
Set the auction date and time plus the venue (venue is optional)
Save the changes
Base will push any for sale listings marked for auction through as auctions to external portals.
You will be able to add multiple Auction dates to the listing should you wish