Surface Pro 5 - Headphone Jack not recognized as Input Device

Hello, I have a Surface Pro 5 (1796 i5) running Windows 10. I'm trying to use the headphone jack as an audio input using an analog audio output from a Fender amplifier. My understanding is that this jack can be either an input or output port. When I go to Sound Settings in Windows 10, it does not recognize the plugged-in audio line as an input.
What I'm trying to do is use the amplifier output to record the sound on the Surface Pro. Suggestions?