The Dissending for Sweets side quest in Hogwarts Legacy can be obtained by talking with Garreth Weasley outside of the Great Hall. In order for this quest to show up, you’ll first need to have attended your first potion class. After that class, you may also need to speak with Garreth in the classroom. When you’re ready, speak with Garreth and he’ll ask you to retrieve some dried Billywig sting from Honeyduke’s cellar in Hogsmeade.
Find the statue of the one-eyed witch
Head to the Faculty Tower Floo Flame in the South Wing then head towards the Grand Staircase. The statue will be on the left of the alcove before the staircase. Interact with it to head into the secret passage.
Explore the secret passage
Follow along the secret path until you reach the lift. Cast the Reparo spell to repair it, then use the lift (make sure to grab the chest behind it first). At the bottom of the lift, grab the chest behind it then proceed following the path. Take the first left and use Incendio to burn away the spider web. Continue on grabbing the chest then ending up on a platform. You’ll need to use Levioso while standing on it to raise the platform and then climb up the wall.
This should bring you to an underground cavern. In the water below, you can find a couple of chests. When you’re ready to progress, head back up to the wooden platform and use Levioso on the adjacent platform to cause it to rise. Jump across then use Reparo on the next platform. Follow that up with a Levioso, then burn the spider webs blocking your way to proceed.
At the next wooden platform, you’ll need to repair an adjacent platform to fix it. If you try to use Levioso, the platform will be blocked by some metal bars blocking the way. To the left of the bars, there will be a handle. Use Accio on it to raise the bars then follow that up with a Levioso to raise the platform. Follow the path to reach the gate leading to Honeydukes. Cast Incendio the braziers on both sides of the door while standing on the platform to open it. Before heading in make sure to loot the chest to the left of the door in the water. You can then use the ladder to reach Honeydukes.
Find the Billywig Stings
In Honeydukes, loot the chest under the stairs then head into the storage room. Follow it around to find the Billywig stings on the table across from the stairs.
Rewards
Deliver the item to Garreth. This will complete the quest and you’ll receive a Quidditch Board for your troubles. If you request to be compensated, you’ll receive 300 gold!