The Hippogriff Mark’s the Spot side quest in Hogwarts Legacy can be obtained from Poidsear Castle Floo Flame on the Poidsear Coast. Head there and deal with the poachers. There will be a tent on the west side with a map on the table. Grab the map then travel to the far southern tip of Manor Cape.
There you will find Henrietta’s Hideaway. Enter the hideaway and use a fire spell on the brazier under that statue on the left side of the room. This will reveal a cube that you’ll need to move on the platform in front of the door. Cast a fire spell on the left cube and ice on the right to open the door. Head through the doorway to find some Ashwinders lurking about. Deal with them then look at your map.
It will indicate which braziers under the Gryphon statue should be lit. Make them match then loot the chest behind the Gryphon to receive the Treasure-Seeker’s Gloves appearance completing the quest.