Make PulseAudio (or alsa, ...) recognize 3.5 as input (& not headphones)

I want to record my guitar by connecting the 6.3 output of my amp or pedal to the 3.5 audio (preferably S/PDIF if that's superior) input/output of my LapTop on Ubuntu 17.04. I got the required cable and have tried a couple of things but failed so far.

How can I get Ubuntu to recognize the sound input as such so that I'm able to record?

pavucontrol lists Headphones as plugged in but microphone to not be plugged in if I use the standard jack.