You need to grab fire extinguisher, go to the shelf near the cash register as shown in video, crouch, tap S once, and press Space to climb, also hold S+D. When you can't see your left hand above the screen, you hold left click. Do this as early as possible to get the best result. If you can see the key clearly, but cant grab it, try to spin the camera near the blade of the key, use fire extinguisher if you think it'll help. If you still cant grab it, reload checkpoint and try to do that again. Works on any fps, its a bit easier to get on 30fps.