I try not to use Windows, so apologise when my answers are not 100% correct.
You are aware that PuTTY is not supporting Secure Cards out of the box ? Only workaround is, that PuTTY is using ssh v1 protocol “ChallengeResponse” for authentication. (See 4.23.4 of PuTTY documentation ).
There are older PuTTY Versions SC and CAC that also support Hardware Tokens, WHEN you have the right libraries etc.
NK is suggesting to use GPG’s agent that has a build-in support for PuTTY. So this (GPG and the PuTTY Support ) needs to be setup first like described. Also you will need to extract the public key out of the NK Pro and add it on the server side to your authorized_keys.
The use of the NK Pro is then total transparent for you inside PuTTY. You could easy check that with to ssh connections: first open a ssh connection with the NK Pro plugged in, then close the ssh session , unplug the NK Pro and try to re-open the ssh connection. It will (should :D) not work.
I am using ssh direct on my macOS Terminal with zsh. You could also try to use the MS Builtin ssh client instead of PuTTY. “Pure” ssh might be much easier than a GUI client above level. To use the NK Pro , you will need a pkcs#11 library. Have a look on https://www.nitrokey.com/documentation/applications#a:general e.g. for the setup of OpenSC ( or again GPG with an agent).