Update a Genesys Cloud user's presence status via the Users Presence API

Steps

1. Obtain an OAuth2 token via the client credentials grant: POST /oauth/token on the region's login host (e.g. login.mypurecloud.com) with a Basic auth header (base64 client_id:client_secret) and grant_type=client_credentials, then use the returned bearer token on subsequent calls.
2. Call GET /api/v2/presencedefinitions (paginated with pageNumber/pageSize) to list organization presence definitions and find the target definition's id by matching its systemPresence value (e.g. "Available", "Break") among entries flagged as primary.
3. Read a user's current presence with a GET on the user (expand=presence) to see the active presenceDefinition and source.
4. PATCH /api/v2/users/{userId}/presences/purecloud (or /presences/{sourceId} for a non-PureCloud source) with a body containing presenceDefinition: { id } to change the user's presence to the desired system or custom status.
5. To change another user's presence rather than your own, ensure the calling OAuth client/user has been granted the presence:userPresence:edit permission.