Marius Masalar, at his blog:

“Hey Siri, Add a new Writing task”
This example creates a new task in Things within my ‘Writing’ area, assigned to Today, and presents me with a pre-filled quick-entry window where I can add the title and hit save.

I’ve already created almost 50 Siri Shortcuts, but I haven’t done much in the way of integrating Siri Shortcuts with Things. I’ve only set up a couple Siri Shortcuts to filter my tasks with particular tag/list combinations. Follow the link above, though, for some really good examples of Siri Shortcuts set up to create tasks in Things. The post inspired me to set up a few.

If you’re new to Siri Shortcuts, one method is to create them from within the suggestions in the Settings app (Siri & Search > All Shortcuts). Suggestions typically show up there only after you’ve taken an action on your phone. For example, once you use Maps to navigate to an address, that address will show up as a suggested shortcut

For Things, a better method is to create the Siri Shortcut from within the Things iOS app. In Things’ settings, you’ll find a “Siri & Shortcuts” section. From the “Create Shortcuts” area you’ll find the option to both create Things tasks with shortcuts, and view tasks filtered to your choosing.

Things Siri Shortcuts screens