This article gives you the guideline to setup how to configure webhook in Datadog.

Pre-requisite: Setup webhook
Create the webhook
Go to the Webhooks integration tile and click to create the new webhook


Create a new webhook. Fill name, url (Cloud Alert event collector), payload as well.

Step 1: Define the rule
Eg. You can simulate the alert by configuring test notification
You try triggering Event crosses a threshold by selecting state transitions of monitor to send notification to your channel.

Step 2: Define the metric
To start configuring the monitor, complete the following:
[Monitors] menu > [New Monitor] > [Metric] monitor type
In [Define the metric], select a metric > define from ... > Specify metric aggregation: e.g: avg by (group by)

Set alert condition: if ( Alert Threshold > actual) or (warning threshold > actual) >> Warning Alert

Step 3: Set Actions to Notification
Fill the created webhook in the [Notify your team] with Title & Content of notification


Output
Once you setup webhook successfully, Datadog alert event will be ingested to Cloud Alert.

Reference: https://docs.datadoghq.com/integrations/webhooks/