This achievement is obtainable as part of the optional objective "Sedate the Bank Personnel (Look for the Pharmacy)" in mission 3; The Bank Job.
First, you need to head to the pharmacy (objective marker). Once there, read the "Justice for Cienfuegos" note on the door, then find Teresia Cienfuegos in the building above the Black Market (easily locatable - notice the updated mission marker), talk to her, then head to the auction across the street (updated mission marker).
The aforementioned steps appeared to be mandatory for me to be able to participate in the auction. Prior to talking to Teresia Cienfuegos, both gates to the auction were locked.
Upon arriving at the auction, use Semblance on one of the guests (I used the second guest I came across after going through the now-open gate, the first after the next door that cannot be closed), then just sit down on one of the chairs at the auction (I did not have to hide the unconscious body, as it fell behind some cover), and take part by pressing
when prompted to. Personally, the potion cost me 380 coins, even though my last bid was 400 coins. Before participating in the auction, I had 1100 coins.
The achievement unlocks immediately, but don't forget to pick up the Poppy Tincture you just purchased.