If you know the tile, you know its X and Y coordinates.
You can convert between tile positions (px,py) and layout co-ordinates (x,y) using the PositionToTileX/Y and TileToPositionX/Y expressions.
There must be something that is specific for that tile. You have 'painted' it on the map.
On the moment that you are 'painting' tiles, you see numbers in the tilemap bar. That is the ID of that tile (zero based). Once you painted a "square" on the map. I gets a position, in the form of numberd positions. (forgot if its zero based, i suppose so) Positions are like 4th sprite from the left, 2nd kolom. Or px=4, py = 2.
The condition > Tilemap > Compare tile at px=4 py=2 will give you the ID of that tile. The ID refers to wich square you painted in there.
If the ID matches the square that you want to pick. Translate its px,py to real layout x,y coordinates.
You can ofcours loop true the tilemap with a px and py loop and compare them one by one.
So far for the system you asked to explain. But in the end you dont seem to care that much.
Your tile is at px=2, py=1.
So if you place on object on the x,y of that tile you set position of that object to
(for x) Tilemap.TileToPositionX(2)
(for y) Tilemap.TileToPositionY(1)
But, i told you that allready.