I'll be honest, I think this glitched on me in a good way.
Right after I talked to the bartender (the first conversation in the game), I was trying to figure out how to interact. I pressed
... and then I pressed both together and this popped. And my game promptly crashed (in that I couldn't do anything and had to quit to dashboard).
I tried it on an alt, same thing. Pressed
, popped achievement. crashed game.
The ending shows as having registered at the menu screen. Also, the achievement for completing any ending popped when I loaded back in, so I didn't miss out on this.
Sorry this doesn't give the actual directions to how this works, but it does work, and it seems to register, so that's the good thing.