According to the documentation, it is not possible to get all onlineMeetings and the $filter query parameter must be added to queries, so you can use the /events
endpoint to get all events. Events with isOnlineMeeting set to true are onlineMeeting.
All events with isOnlineMeeting value true can be filtered through $filter, but unfortunately, this property does not support filtering, so you can only get all the events first, and then loop through all the events in your code to filter out onlineMeeting.
GET /users/{id | userPrincipalName}/events
See this link for details on listing all events:
https://learn.microsoft.com/en-us/graph/api/user-list-events?view=graph-rest-1.0&tabs=http
Hope this helps.
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