If you want the sprite truly anywhere on the layout, you can set the position to:
random(layoutWidth)
random(layoutHeight)
Unless your layout is entirely open, as in no solids or areas sprites shouldn't be (walls, for example), you probably don't want this.
If you want to do more of a concept of spawning zones, I would make spawn zone sprites. Set the sprites to invisible, so that you can see them in the editor but not while playing.
In the condition part of your action, use the System condition (select System, not the sprite), and use "Pick Random Instance". This will pick a random spawn zone sprite each time. (if new, you might want to read about "picking" in events)
If you want them in random spots in the spawn zone, you can use the same concept as I started off with, using set position to:
random(spawnZone.BBoxLeft, spawnZone.BBoxRight)
random(spawnZone.BBoxTop, spawnZone.BBoxBottom)
I do a similar thing, except for movement instead of spawning, in this game: loubagel.com/games/sunnyside-smash
I wanted the characters to move around randomly, but there are areas sprites can't go - in the water, in solid objects such as houses, and areas I don't want them to be such as right in front of the house doors. So I made a sprite object using the same concept I explained above. I covered the map of areas I wanted them to go with instances of this sprite. The nice thing was that they didn't have to be the same size - if there was a big wide open area, I could stretch one instance of this sprite over it all.
When a character is ready to move a random "move zone" sprite is picked. Then, a random coordinate inside that sprite is picked. Then I just use the pathfinding behavior to move them to that coordinate.
I know the example is a bit off topic, but hope it helped.