You’ll receive the A Demanding Delivery side quest automatically when you start the Complete the First Keeper Trial main quest. Parry Pippin will send you an owl post asking if you have time to help out with a delivery. Make your way over to Parry’s shop in Hogsmeade and speak with him. He’ll ask you to deliver a couple of invisibility potions to Fatimah Lawang in Keenbridge. Agree to help him out then travel to the village south of Hogwarts.
You can find Fatimah at her stall across from the Floo Flame. Speak with her to deliver the potions. She’ll respond by asking you to drink the potion to prove it works. Even if you choose that you’d rather not, she will still insist that you drink the potion so go ahead and just do it.
When you’re done, return to Parry and let him know you delivered the potions. If you wish to ask for an additional fee, he won’t be happby but will give you 500 gold plus the Potioneer’s Cap appearance. If you chose to just hand it over, he’ll give you 300 gold plus the Potioneer’s Cap appearance.