On the web site, go to the day you booked in your itinerary and click "Add Plans" at the top left, then select "Add a Note". Or, on the app, click the plus sign and then the hat with stars coming out of it, and then "Notes" at the bottom.
You'll be able to select the location (Animal Kingdom) and put in a description of what it is. I put "
DVC Safari" as the short description and the confirmation number in the details box.
It'll then show up in your MDE itinerary.