This was previously glitched but has since been fixed.
Currently no offering can be unlocked via the Bloodweb to increase your chances of getting this map.
You need to be playing on the Dead Dog Saloon map in the Grave of Glenvale realm. (Currently only map for this realm.)
Then locate the saloon building, it's extremely hard to miss as this is possibly the smallest map in the game. It's a giant building with two staircases in the back. It has another staircase inside along with some tables, a bar counter and a piano on the first floor.
A map unique feature is this building has breakable walls so be weary of those.
The gen is usually located outside on the second floor balcony. (At least this is the only place I've seen it.) It does have two different spots on this balcony, each possible spawn spot is closer to the opposite escape drop hole spots.
You can see the saloon in the image below.