That's an impressive little beast, especially at that price.
I had a quick Google and you just need to pulse the focus pin to GND (I'd expect 100ms would be a good). I'd suggest using a digital in, transistor out optocoupler between the microcontroller and the camera. That would be the minimum number of components to wire up while being pretty much bullet-proof in terms of camera safety.
It looks like Nikon use a variety of proprietary connectors so I'd buy a cheap shutter cable release to match the camera and then cut the cable off to get the connector and cable.
- adamhanlon likes this