You’ll need to check out the app to see if it has been updated for Siri Shortcuts, or see if the garage door opener can be used with the Home app.
Either of those will let you do what you want.
If the app has been updated, then after you’ve used it to hopen the door a coiuple of times you should be able to find the shortcut undser Settings > Siri & Search > Suggested Shortcurts (show all if necessary). When you tap the + button at the right it will prompt for the activation phrase.
If you can add your opener to the Home app, then you would create and manage the shortcuts there. Since I don’t use this app (no devices) I don’t know the specifics; but it shouldn’t be hard.
If the devcie/app can’t do one of those, then it is not capable of using the Siri Shortucts. All you can do is bug the manufacturer to up their game.