Border's shape of the boxes (in supersource)

I'm facing a challenge with the ATEM 2ME switcher.
I would like to change the border's shape of the boxes in the supersource. I would like rounded borders.
I tried add borderRadius in the xml langage of the supersource but it doesn't work.

Do you have any idea how to do that?

sorry, only square corners in SS.

you can get a rounded corner look if your art is built with openings that way and used as a foreground