As you know if you accessed any menu, tab, or map in Assassin's Creed: Origins since Jan. 23, the new DLC 'The Hidden Ones' is now available. The beginning quest, aptly named 'The Hidden Ones,' will autopopulate as long as you have (1) finished the main game, (2) completed the very-hard-to-miss 'Incoming Threat' mission, and (3) purchased the DLC.
Launch the mission proper by following your marker to the ferry in Memphis, and talking to the ferryman.
Hit continue to complete the mission, and begin your journey to the Sinai.