I use socat along with npiperelay to allow the ssh client in my WSL sessions to access the Windows ssh-agent.
Similarly, you can also do this with KeepassXC
Edit: These directions are very similar to those linked in the comment this is in reply to and either will work.
https://gist.github.com/duebbert/4298b5f4eb7cc064b09e9d865dd...
https://github.com/ndbeals/winssh-pageant
I use both this and rupor's wsl-ssh-agent from the other thread extensively.
I was in need of a solution to this issue for a long time before ndbeals thankfully released this about 9mo ago. And I was checking around the web regularly! I nearly wrote my own as well.
The use case for me was to allow WinSCP and Sourcetree to be used with the Windows ssh agent.