"The Microsoft Graph .NET Client Library doesn't implement any authentication by default. Instead, you will need to retrieve access tokens for the service via the authentication library of your choice or by coding against one of the authentication endpoints directly."
https://github.com/microsoftgraph/msgraph-sdk-dotnet/blob/dev/docs/overview.md
"The recommended library for authenticating against Microsoft Identity (Azure AD) is MSAL."
https://github.com/microsoftgraph/msgraph-sdk-dotnet