This kind of thing is easiest to do with the Listener, I think.
Go to [Decker > Listener] in the menu, type the snippet of code and then use Shift+Enter to run it.
(Or you can just put the script inside a button and click it in interact mode, if that's easier!)
For anyone finding this post later, this is about the rounded corners you see at the edge of decks published online.
You can use deck.corners:0 to remove the rounded corners from your project (this works by setting them to pattern 0/transparent) or you can use another pattern index number instead of '0' to change the corners to that color or pattern instead!