You'll need to record a trigger phrase - such as "Ok Google" - since it's apparently unable to pull in a pre-existing voice command you may have already set up on another Assistant-powered device.
In some ways, this additional and inconvenient step defeated the convenience of a digital assistant in the first place. You could also use Siri Shorcuts to run Google Assistant routines, which are like multi-command scripts.
Though this might sound complicated, once you get the hang of Apple's Shortcuts app, setting up this sequence is relatively simple. What's more, support for Siri shortcuts also lets you talk to Assistant hands-free on an unlocked device by uttering the same command (if that's the one you pick).
Screenshot of the app via The Verge
The new shortcut integration is available by updating to the latest version of the Assistant for iOS. They no longer need to open the Google Assistant app to trigger the feature. However for those who want the best of both ecosystems it's a whole lot better than having to open up the Assistant app manually and then start speaking to Google's service. When you open the app, you should be prompted to add "OK Google" to Siri. As such, you are not confined to using the traditional "OK Google" or "Hey Google" commands - you can make it anything you like.
Bottom line, this is an objectively awesome Costco deal that Google Assistant enthusiasts don't want to miss out on. That's just a sample shortcut that various tech blogs have come up with.
With the introduction of support for Siri Shortcuts, today Google has made it easier to accesses Google assistant just by saying "Hey Siri" and then followed by "Ok Google". If you used the same name for your Shortcut, you could simply say, "Hey Siri, going to bed", which will call on the Assistant Routine and perform the action.