If purchasing tickets for Halloween and Christmas parties (for example) the discount (if eligible) is applied when booking online as long as your DVC affiliation is noted in the MDE app. I booked Dive Quest for my husband the other day and did Keys to the Kingdom in September and for both it automatically applied my DVC discount when purchasing online. I currently don't see any dessert party discounts.
So, it just depends. If you read that there is a discount for an experience/tour and can't figure out how to apply it when booking online, then I would call member services.