Standing on the platform in the room below, jump up (crouch jump makes it easier) and press right against the door frame through the transition. After the transition, perform a spring ball jump mid-air to reach the ledge.

Entrance condition:

{
  "comeInWithPlatformBelow": {
    "minHeight": 7,
    "maxHeight": 7,
    "maxLeftPosition": 1,
    "minRightPosition": 1
  },
  "comesThroughToilet": "no"
}

Requires:

"canCrossRoomJumpIntoWater"
"canTrickySpringBallJump"
{
  "or": [
    "canCrouchJump",
    "canTrickyJump"
  ]
}