The GraphQL endpoints use the same authentication as ODP's REST APIs. The private key must be used for all GraphQL queries.
The hostname for the API is region-specific. Refer to the list below to find your region's hostname:
Use the GraphQL API to query real-time segments for customer events within the last 28 days. If you want to use data greater than 28 days old, use the standard segments on the Customers > Segments page (see Create segments).