We’ve been talking about this in the existing thread on this feature. It looks like at present there is no way to do it officially except by either talking to it or using the Amazon app.
However, it is possible to use SmartThings to have one Amazon device speak the words and have a second Amazon device react to that. Or you could definitely use a non-Amazon text to speech device in this fashion. It’s clunky, but it can work.
Updated to add I just realized you can use the two device method with Amazon’s own Geolocation option in their own routines without needing smartthings for this. You have to use the Amazon Alexa app on your phone then create a routine there for when your phone comes and goes. Have that routine speak the activation phrase on one echo device at your home so that another echo device at your home can hear it. Geolocation was only added to using Amazon routines a few months ago, but it is there. 
Using SmartThings with the virtual sensor method would allow you to set up a lot more filters on when it happens, and the Amazon-only method is not going to work well for a lot of multi person households, but this is an option for simple use cases.
Here’s the existing discussion (the thread title is a clickable link)
Amazon Guard Available yet? (May 2019)
Other than that, we’ll just have to wait until Amazon makes more options available.