Copy external dlls to final module directory

[MartinPerry]

After the nanobind_example is build, I want to copy some external dlls to the final destination folder. However,

add_custom_command(TARGET nanobind_example_ext POST_BUILD 
               COMMAND ${CMAKE_COMMAND} -E copy
               "D:/my_folder/some_lib.dll"              
                $<TARGET_FILE_DIR:nanobind_example_ext>)	

is not working. How to fix this?