Adding discount to an event can be done by first creating a product via setup (called 'discount' for example). Give this product a negative amount (e.g. € -12,00).

Now add the product to an event. The discount rate (and possibly the product name) can always be changed manually in the event.

Since the discount is added as a product to the event, the event's total price is adjusted accordingly, and subsequently you can find the discount in the quote and invoice.

Please make sure that you place the discount rate below the correct VAT rate so the discount is properly reflected in the cost overview.