Right now you can only create triangular windows using the Magic cube.
First of all, you need to place a window that will become a lower part of the triangular window you're creating. Then use the Magic cube to draw the triangular part of that window on top of it!
Here's the video dedicated to it.
We're using the Web version on that video. However, main controllers are the same for all apps, so you won't have any difficulties reproducing these steps on your device.