I used a cable like this: https://www.amazon.com/flitecordr-10-feet-Recording-Smartphones/dp/B01NAQHYRG it plugs into your smartphone.

Alternatively Nflightcam makes one as well but it's much more expensive: https://www.amazon.com/Nflightcam-Audio-Solution-GoPro-Hero5/dp/B01MT3Q50Y/