you need to do custom nights and secret dommy daddy golden freddy variant.
Viewing post in Stag Nights At Bucky's comments
It's almost done so I'll add him at some point before the "final" update, but there's already a custom night in the extras menu (it's available from the start, you don't need to finish all the normal nights first)
Next update will likely add him, general improvements to the map like having the storage room not be a white box, more animations, etc.