Standing on the platform in the room below, jump up and press right against the door frame through the transition.

Entrance condition:

{
  "comeInWithPlatformBelow": {
    "minHeight": 6,
    "maxHeight": 6,
    "maxLeftPosition": 1,
    "minRightPosition": 2
  },
  "comesThroughToilet": "no"
}

Requires:

"canCrossRoomJumpIntoWater"

Dev note: This strat only works for platforms that are 6 tiles below the door transition (which is the closest distance that occurs in the vanilla game). A crouch jump can be used but is not required.