Dexcom wasn’t able to get me a G6 transmitter before the one I had expired (due to mistakes at Dexcom). I had no choice other than hack the transmitter with an iOS app called Spike. Unfortunately I don’t think it’s available for the moment because of a dispute with Apple. Check Spike-App.com or their facebook page to get an update.
I also found this process, but it’s cumbersome and I haven’t tried it.