you will need to have a column in the cart for the category, in the add to cart button, for the accessories, set the category column value to Accessories.
in the discount rule, set the triger to use Total number Of Items With A Specific Column value:
If the total Number of Items where 'Category' has a value of 'Accessories' is '>' a value of '0'
and set the calculation to use Based on Items with a specific value:
Total of 'totalPrice' where 'Category' has a value of 'Accessories' 'times' '0.20'