Documentation of the Google Assistant SDK says that you need to hit enter to talk to the assistant. Is there a native way to set up a wake word?
Not yet. Remember this is just the first Alpha release. As noted in the release notes, "Library with hotwording capabilities" is "Coming Soon".
However... this does mean that you have the flexibility to wake up the assistant based on whatever you want. The sample makes you hit enter, but you can set it up to trigger based on other things. Consider, for example, turning on the microphone when someone opens the door and after a welcome greeting is played.