How to use ffmpeg to replace part of an image with a transparent polygon

I would like to replace part of an png image with a transparent polygon. I have tried to use the ffmpeg tool in mac terminal.

I have tried to following command. But this draw a black rectangle and not a transparent one: ffmpeg -i hemma_bil_2200_crop.png -filter_complex "crop=1344:1200:50:50, drawbox=50:50:100:200:#00000000@1:fill" output.png

1. I aim to get it transparent and not black
2. I would like to draw a polygon instead of a rectangle if possible. The polygon I would like, is a rectangle with the right side somewhat sloping i.e. the base should be somewhat longer than the top.