how to connect 3.5 mm headphone out into a macbook air as a mic input

I have a fender amp with a 3.5mm headphones out. i would like to find some way to connect that into my 2017 macbook air so i can record my playing into garage band or logic pro. I tried to plug in a TRRS male to male cable but my mac treats it like headphones and keeps using the inbuilt mic. I got an external mic hooked up and working in the form of earphones with a built in mic, so i purchased a combo audio adaptor from sennheiser that had a separate mic in and headphone out ports combined into one male 3.5 TRRS, however my mac would not detext an external mic that was plugged in through it. is there any way to fix in settings, or any adapter i can buy that will work? i am on mac os 10.14.2

thanks