So, to clarify: The "Skyburners Command Beacon" can drop from any yellow health cabal enemy on the Dreadnaught. Once you receive the Beacon, you can activate it on terminals aboard the Cabal ship. After you activate the code, lots of Cabal will spawn, one of them will drop a "Skyburners Deployment Code". Take this code to the cabal treasure area in The Masoleum, activate it at the terminal. Complete the quests for the Code, they vary from killing enemies to scouting locations, the final task will be killing a specific Cabal major, these tasks have a time limit, you must complete them quickly. After you kill this major, return to the cabal treasure area in The Masoleum, you will be able to open the back door, and obtain the premium loot chest inside.