Defeat Pumpklings near Stratus Farm and obtain Pumpkling Juices. Then, talk to Farmer Keifer at Stratus Farm. Use your quest tracker to see your progress.
Farmer Keifer: This is really odd. NakanoAzusa: What is? Farmer Keifer: Those pumpkins are monsters that move around, but Farmer Keifer: they do leave the juice behind when they die. HiiragiKagami: Well, they're originally pumpkins. It's no surprise. Farmer Keifer: But the scent... their juice must be thicker than average Farmer Keifer: pumpkin juice. They must be worth money, maybe! NakanoAzusa: You were treating them like enemies of your lifetime NakanoAzusa: only a minute ago. Farmer Keifer: Yes, but I can't just sit down and lose money. How Farmer Keifer: about this? Please sell the pumpkin juice to me. Unless Farmer Keifer: my nose is serving me wrong, the pumpkin juice will be Farmer Keifer: the next big hit in the market.
Reminder Dialog
Farmer Keifer: That smell! They may have turned into monsters, but Farmer Keifer: the quality still remains! With loss comes opportunities!