As of now, you can only use the subscriptions resource to subscribe to the Legacy Alerts resources in Graph: subscription resource type - Microsoft Graph v1.0 | Microsoft Learn
The Legacy Alert resource does not include alerts generated for Microsoft Defender for Endpoint: Use the Microsoft Graph security API - Microsoft Graph v1.0 | Microsoft Learn
This means that there is currently no functionality to create a Graph subscription for alerts generated from Microsoft Defender for Endpoint. For organizations that want to trigger a workflow from a webhook in response to an MDE alert, this leaves no option for an instant webhook.