Do you want to apply it to all bookings or based on a rule? Or just manually added when you need it?
If automatically (to all or rule based), set the criteria on the discount to apply (either all bookings or specific bookings based on. For example, you could do a last minute booking discount by arrival within 10 days out. If you leave the criteria at "all bookings", it will automatically be applied to everything, and you'd need to manually remove it if you don't want it to apply.
If you want to manually add the discount, you don't need to add a Surcharge/Discount record for it. Currently, that section is only for automatically applied items, based on those rules you configure. Eventually we'll allow you to create manual ones there that you can select from the charges grid (like taxes work now), but we're not there yet.
If it's a manual discount, the best way is to go into Charges and add the discount directly there. That grid is fully editable, so if you added a new Rent line item after the last existing Rent line item, with a Description of "Discount" and a Rate of -10% -- that's going to be the best way to add a one-off discount.