Today, January 2025, the resource type exists and can be deployed with:

                        {
                            "name": "[format('{0}/{1}', parameters('automationAccountName'), parameters('webhookName'))]",
                            "type": "Microsoft.Automation/automationAccounts/webhooks",
                            "apiVersion": "2018-06-30",
                            "location": "[parameters('location')]",
                            "properties": {
                                "isEnabled": "[parameters('isEnabled')]",
                                "expiryTime": "[if(not(empty(parameters('expiryTime'))), parameters('expiryTime'), '11-26-2036')]",
                                "runbook": {
                                    "name": "[parameters('runbookName')]"
                                }
                            }
                        }

As you see, the webhook resource is a child resource to automationAccounts.

The isEnabled is type: bool,

The expiryTime expects format: MM-DD-YYYY and is required.

Beware: the webhook Name should be unique inside the automationAccount. Re-deploying with same name will never reveal the service uri (property: .uri) except during first deployment.