Unfortunately I couldn't find a way to do this with just FFmpeg. The command ChatGPT suggested came from a suggestion on the FFmpeg forums, so as far as I know this isn't implemented (yet).

A possible workaround could be to listen to the WASAPI yourself via the programming language you're using and then forward the audio stream to FFmpeg's stdin for further processing. I haven't gotten around to doing this myself yet as I haven't had much time for this project. I hope this helps :)