1 Start to create the trigger based on the watcher you want.
2 When happy with the trigger choose Create New Shortcut
3 in Shortcuts drag the Control My Home item (if you can't find this type "home" in the search bar on the right of Shortcuts, into the Shortcut.
4 This will now show: Set Scenes and Accessories
5 Click Scenes and Accessories, this allows you to chose which HomeKit device you want to control
From there you can configure the device to do what you need.
Don't forget to name and close the shortcut before you switch back to Trypa, you can also add as many items in as you need to control multiple items.