How do I troubleshoot webhooks?

I believe I’ve followed the automation instructions for creating a webhook, but I can’t get it to work. I’m using Home Assistant Core version 0.117.6 in a virtual environment. have the following in my automations.yaml:

- id: '9'
  alias: whtest
  trigger:
  - platform: webhook
    webhook_id: whtest
  action:
  - service: input_boolean.turn_on
    entity_id: input_boolean.camindoorwifi_motion_detected
  mode: restart

I’ve tried to run it by using curl, which returns a 404 error.
Curl -X POST http://ip:port/api/webhook/whtest

Here’s the output from curl:

*   Trying 192.168.1.16:8123...
* TCP_NODELAY set
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0* Connected to 192.168.1.16 (192.168.1.16) port 8123 (#0)
> POST /api/webhook/whtest HTTP/1.1
> Host: 192.168.1.16:8123
> User-Agent: curl/7.68.0
> Accept: */*
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 404 Not Found
< Content-Type: text/plain; charset=utf-8
< Content-Length: 14
< Date: Fri, 13 Nov 2020 14:53:02 GMT
< Server: Python/3.8 aiohttp/3.6.2
< 
{ [14 bytes data]

100    14  100    14    0     0   1400      0 --:--:-- --:--:-- --:--:--  1555
* Connection #0 to host 192.168.1.16 left intact

How do I figure out what I’ve done incorrectly, or missed?

Apparently it is some kind of permissions error. The webhook works from a different machine.