Webhook URL rejected
HTTP status: 422. Code: webhook_url_rejected.
What it means
Hometown does not deliver to this URL. A webhook URL must use HTTPS on port 443 or a port from 1024 to 65535. It must resolve to public addresses only and answer without a redirect. The detail field says which rule the URL broke.
How to fix it
Correct the URL as detail says, or point it at a public HTTPS address of yours, then send the request again.