I would guess that you need to add the opencv_world451.dll to the system/user path. You can do this through the gui shown in the link from your original post by adding the path to opencv_world451.dll to the path variable (probably something like C:\Users…\opencv\build\x64\vc15\bin). Note: if you add to the system path you may need to reboot your system in order to see the change. If you add to the user path then you will simply need to re-open the application/cmd etc. which you are building from.
Alternatively you can launch the command prompt and enter
set "path=C:\Users\<ACTUAL_PATH>\opencv\build\x64\vc15\bin\;%path%"
and then launch your application from there.