They are both correct. They have slightly different connotations, though, and
the first construction would be much more common.
“She put a pen on top of the fridge.” This simply means that she had a pen
in her hand, and she reached up and set it on the top of the refrigerator.
If you say “she put a pen on the top of the fridge,” you are describing the
same actions. But the emphasis is a little different. “On top of” simply
specifies the relation between the pen and the fridge, telling you that the
pen is above the refrigerator, and in contact with it. Gravity keeps it in
place after she places it there. “On the top of” instead is emphasizing
where on the fridge she put the pen. That is, she put it on the top, rather
than on the side or on the front of the fridge. (Presumably held in those
places by a magnet instead of by gravity.)
https://reurl.cc/mrvllj
There isn't much difference between on top of the fridge and on the top of
the fridge. Perhaps "on top of..." is less likely to be attached to the top,
but rather just resting or stored there. On the top of the fridge might refer
to a scratch in the paint.
https://forum.wordreference.com/threads/in-back-of-vs-in-the-back-of.2579898/