Summary:
You can get this by app-only authentication method. Get an app-only access token and use it for authentication.
I am also struggling on a similar type of question and I need to retrieve planner tasks. For planner tasks app-only scope is not supported but In your case I think it should work
Read both these links for more details.
https://graph.microsoft.io/en-us/docs/authorization/app_only
https://graph.microsoft.io/en-us/docs/authorization/permission_scopes