On screen rect that shows my display size?

I’m making a game with a large, tiled playfield, and a scrolling “portal” that moves over it. Is there any way for me to specify the “portal” rect size in Tiled, so I can get a sense for how much of my playfield the player will be able to see at any given time?

I’d just make a transparent “cut-out” image of the portal like this, and put it in an image layer. Then it’s easy to move it around, or hide it.

Thanks! Works great. (Though figuring how to add an image to an Image Layer was tricky! Answer: the properties sheet.)