Enter the outhouse and examine thetoilet bowl. You should see an "Investigate" prompt on screen. Interactwith the prompt, and Henry will retrieve the Royal Treasury Key from the bowl.
Be aware that retrieving the key from the outhouse will make Henry dirty. It's highly recommended to wash up at a trough before proceeding withthe rest of the quest. Talking to NPCs while dirty can lead todifficulties in dialogue checks and interactions.
Once you have the Royal Treasury Key and are clean, return to Zizka and Rosa to continue the Oratores quest. You'll then explore the undergroundpassages with Samuel. Be prepared, as this quest is quite lengthy andinvolves several more stages.