The construction that you have created should indeed work. So perhaps double-check:
Have you published before testing the webhook?
Are you certain you are posting to the correct webhook?
Have you correctly specified that the data sent is JSON formatted (the ‘content-type’ header)?
Assuming you have done all these; could you perhaps send me a DM with the instance you are working on? Then we’ll take a look at the logs to see if we may have stepped on a bug here.