You don’t need to create variables. They’re already present, as tags.
To get the tag, click on the tag icon on the right of the flow card’s input field (it’s blue at the screenshot below).
Then, a selection list appears.
Just enter the (first part) of the sensor’s name, to have the list filtered to the sensor’s tags:
So, when your sensor is called Hallway Motion, just enter that to the search bar shown.
Examples:
