According to GoPro documentation, the new budget GoPro Hero is not compatible with external microphones and the GoPro microphone adapter that is available for use with the GoPro Hero 5/6 Black. This has turned out to be false and external microphones and the microphone adapter is indeed compatible with the new GoPro Hero (2018).

To Use a microphone with the new GoPro Hero simply plug in the microphone adapter to the USB-C port on the GoPro Hero. Plug a 3.5 mm microphone into the jack on the adapter and start recording. No need to change any settings, the GoPro Hero 2018 will automatically detect the external microphone and disable the onboard microphones. Like the Hero 5/6 quick capture does not support external microphones. You need to power on the GoPro first, then press the record button.

You will need to purchase the GoPro microphone adapter and a 3rd party microphone. I recommend using Rode microphones with the GoPro 2018, either the “VideoMicro” or the “Video Mic Go”. They are a good quality microphone for the price and work well with most GoPros, Smart Phones and DSLR/Mirror-less cameras.

Rode Video Micro: https://amzn.to/2IvVTX2
Rode Video Mic Go: https://amzn.to/2H0Z38o
GoPro Microphone Adapter: https://amzn.to/2IuVFPV