I did a quick test with Paw (a native Mac API tester, same sort of thing as Postman). Sending the items as JSON as described in the documentation gave me a successful result.
The widget I set-up for this test was updated to show “bar” shortly after the request was sent. I subsequently repeated this with other updates, with similar results; only the time to update the widget varied, which is as expected given how they work.
From your screenshot above it looks to me like you are mixing up the JSON with the query parameters. I would expect you to be using the Body tab in Postman, and if memory serves, then you would enter the JSON as “raw” data in a JSON format.