You would need to use a combination of @RightHand ‘s suggestion and @JohnHoke ‘s suggestion.
Create a virtual switch in smartthings which then triggers whatever you want to have happen in smartthings, maybe run a smartthings routine. ( if you need to have the switch get turned off again, then just have that same routine turn it off.)
Now create an Amazon routine using the Amazon app, that turns on that switch whenever you say your preferred phrase.
Some words may be reserved for echoes own use, so I’m not sure if you can have a routine called “open Michael’s garage door” If the word “open” is an echo reserved word. I just don’t know.
But you should be able to find some synonym that you like. Maybe “enter” Instead of “open” and “shut” instead of “close.” You may have to experiment a bit.
The advantage of Amazon routines is that you can use any phrase you like that isn’t a reserved word. The disadvantage is that you have to say exactly that phrase, it’s not a natural language interface like other echo commands.