To communicate between WSL and the 1Password SSH agent, we'd need to use npiperelay. This tool allows WSL to communicate with Windows' named pipes.
To install it, we need to open the GitHub repository and download the latest release. At the time of writing this post the latest release is v0.1.0 from July 2, 2020.
Unzip it and paste the npiperelay.exe file in any folder that's configured in your system's PATH. For example %USERPROFILE%/.wsl/mpiperelay.exe and then add %USERPROFILE%/.wsl to your PATH.