You need to loop through all tiles, but it will be difficult to pick a random tile in a loop. So the easiest way is to put invisible "spawner" sprites. Then pick a random Spawner using "System pick random instance" action and spawn your object.
To create spawner sprites:
On start of the layout
For "x" from 0 to Tilemap.Width/tile_width
For "y" from 0 to Tilemap.height/tile_height
..Tilemap compare tile at (loopindex("x"), loopindex("y")) = 32
....Create Spawner at Tilemap.TileToPositionX(loopindex("x"), loopindex("y")), Tilemap.TileToPositionY(loopindex("x"), loopindex("y"))