“Cayde wants a race beacon placed on the Dreadnaught.”
“Place Race Beacon ” is an objective in the mission Dread Patrol in Destiny: The Taken King
Drop down to the ground from the scouting and start toward the right-hand side of the Cabal Ship. Head through the passage beside the ship to reach the Mausoleum. Just head straight inside.
The beacon goes underneath where the rupture was. You will have no problem finding the place for the beacon because it is just inside it. After that, it will activate.