You are too funny.
You are right but you ask another question because PATH variable is not set
OPENCV_DIR is used by cmake not windows .
Note
On Windows plugins must be linked with existing OpenCV build. Set OpenCV_DIR environment or CMake variable to the directory with OpenCVConfig.cmake file, it can be OpenCV build directory or some path in the location where you performed installation.
If you don’t use cmake you cannot see this message:
-- You might need to add C:\tmp\install\opencv\x64\vc17\bin to your PATH to be able to run your applications.