I built this to use my Flightcom headset with my FT-817, it will also work with the 857 and 897. The 857 is a little annoying due to the headphone jack being on the back of the radio and the mic jack being under the control head.
Very simple design, I used Cat 5 networking cable to go to the mic jack, and some Radio Shack shielded audio cable for the headphone side. The .206″ mic jack for the headset can be expensive, I got mine at a surplus store a few years ago. There are two switches, one is a momentary pushbutton and the other is a toggle.
These are the relevant pins on the microphone jack from the 817/857/897
Pin 3: +5v
Pin 4: ground
Pin 5: Mic Audio
Pin 6: PTT
Pin 7: PTT Ground
The +5v line powers the microphone element, pins 4 and 5 are the audio being returned to the radio. The two switches go between pins 6 and 7 for PTT.
At the time, I was only interested in doing this as cheap and fast as possible. This was the simplest circuit I could find. The screen capture below is from this site.