On this page of the Disco Elysium game guide, you will find a detailed walkthrough of the Find pyrholidon and slurp it task.
In Disco Elysium, the task Find pyrholidon and slurp it isn't complicated - simply acquire some pyrholidon by any means of your choice, then have Harry take the drug.
Pyrholidon can be bought at Roy's Pawnshop after passing the white Electro-Chemistry check. Place a dose of the drug in Harry's hand (left or right), then make him open the container through a dialog option (Hmm... open the cap) and ingest the substance (Slurp it).
After taking pyrholidon, Harrier will immediately lose 1 health point, so make sure beforehand that he won't die as a result.