Using Hound/Cortana hands-free in the background like "OK Google"

On my Nexus 6P, I can (obviously) say "OK Google" to summon Google's voice assistant hands-free, without unlocking my phone or opening an app. I really want to use this with an alternate voice assistant as well. Is there a way to launch a voice assistant app (like Cortana or Hound) on my phone with a summon command, as I can do while in the app (at least with Hound)? Is there an app that will let me do this, or is there a setting in one of those voice assistant apps that I overlooked that would enable me to launch Cortana/Hound with "Hey Cortana" or "OK Hound" from my lock screen? I really used to think that there was a setting on Android to change your default voice assistant, but I now can't find anything like that. Thanks in advance!