Ability to add dynamic values in Alert’s custom fields

Alerts help you detect and address issues before users notice them by proactively notifying you when Azure Monitor data indicates there might be a problem with your infrastructure or application.

In the Details tab at the advance options there is the Custom properties section, if you've configured action groups for this alert rule. At this section you can add your own properties to include in the alert notification payload. You can use these properties in the actions called by the action group, such as webhook, Azure function or logic app actions.

As a part of those custom properties a dynamic value extracted from the alert payload, or a combination of both can be added to the alert notification payload.

Use the common alert schema format to specify the field in the payload, whether or not the action groups configured for the alert rule use the common schema.

For more details here