How do I o use random spawn?

I have two box:

I just want to randonly, the red box born "on start of layout" in a random box. Simple, but i am almost killing myself. And i give up of Google.

Please, help guys. There are no capx to show, just 3 lines stopping a animation.

PS: Explanation again: on start of layout, red box born in any of two box.

Detail, the game will have more then green boxes.

Sorry english.

On start of layout

system pick random instance of box

-- box spawn red box

here is a picture of what LittleStain was saying