Wall jump in the room above, on the left wall of the doorway, immediately before the door transition. If entering from an air room above, simply hold right and jump to reach the ledge. If entering from a water room above, release jump (while still holding right) when Samus is about to reach the ledge, to reduce Samus' hitbox vertically by changing her pose from wall jumping to spin jumping.

Wall jumping expands Samus' hitbox vertically, making it possible to trigger the door transition with upward momentum. Even though the wall jumping and spin jumping animations look the same, their hitbox is drastically different. Passing through the Toilet centers Samus horizontally in the doorway. Failing this trick will likely result in a soft lock.