Custom Extension Overview
UpdatedThe token issuance start event, **OnTokenIssuanceStart** is triggered when a token is about to be issued to an application. It is an event type set up within a [custom claims provider](custom-claims-provider-overview.md). The custom claims provider is a custom authentication extension that calls a REST API to fetch claims from external systems. A custom claims provider maps claims from external systems into tokens and can be assigned to one or many applications in your directory.
