On this page of the Disco Elysium guide, you will find a walkthrough for the Find speed and snif it side quest.
The quest Find speed and sniff it in Disco Elysium is not complicated - you need to find or get speed and then take the drug.
Speed can be found, for example, in the medicine cabinet in Klaasje's room after passing the Electrochemistry skill check. Insert a dose of the drug into the hero's hand (left or right), then select the dialog option that will make Harry take the drug (Try some speed).
After taking the drug, Harrier will immediately lose 1 point of morale, so make sure he doesn't lose his life.