UPDATED FOR Odoo 18.0

Although Odoo does want you to identify an existing record in order for the Webhook to work, you don't need to update it, you can execute any code you like.

In this example, I am using the "User" model, and I am providing the "ID" of 1, which I know will always find the Odoobot User.  

The Target Record field is completed with model.browse(int(payload.get('id')))​

Note: It is also possible to simplify this to model.search([],limit=1) and use any Model instead of res.user 

Then, the code I execute just parses the payload and uses it to create the new record, here I am creating a new Contact (res.partner):

name = payload['name']

env['res.partner'].create({
    'name': name,
})

Note that there is no error checking in this example, it will only work if the payload provides the ID of 1 (to find the existing User record) and has a NAME so that I can create a new Contact with that name.

To test, I called the webhook via CURL:

curl -H 'Content-Type: application/json' -d '{ "id": 1,"name": "Ray Carnes (Odoo)" } ' -X POST https://webhooks.odoo.com/web/hook/e63e3705-2fba-4795-98a3-64914be0ba8b

Replace the POST {URL} part with the URL Odoo creates for you