This is the first story achievement of the DLC.
In order to get it, once the quest option goes live (the quest is titled "Envoys, Wineboys"), find a notice board where you can get the information to start the quest. Travel to the town where the knights are waiting and strike up a conversation. You can either participate in or skip a battle with brigands. After that, accept the offer of the knights to take the quest and their offer to start now. You'll find yourself transported to Touissant and the achievement will unlock.