Polygon Mask

I'm having a difficult time trying to do something in Resolve that was quite simple in PP. I want to draw a mask around an object, and have only what's inside that mask show.
An Opacity Mask drawn with the Polygon tool did the trick in PP, but searches haven't yet satisfied how to do it in Resolve. Anyone got a clue?
An Opacity Mask drawn with the Polygon tool did the trick in PP, but searches haven't yet satisfied how to do it in Resolve. Anyone got a clue?