If you are looking to load a workspace-scoped variable to your DAB, you can leverage the lookup keyword. Here is an example of a variable resolving webhook notification called slack-notification :
variables:
webhook_for_notifications:
description: Webhook ID added to Databricks' Notifications where all alerts for the jobs and tasks will be sent.
lookup:
notification_destination: slack-notification
The notification_destination is only one of the supported objects. The full list is available in the documentation.