Register GraphAPI subscription in multi tenant case - microsoft-graph-api

Question

Having an Azure "App registration" which is single tenant it is possible to register a subscription for the graphAPI for calendar events with the following resource:
/users/<user-id>/events
However after switching the app registration to multi-tenancy when attempting to register by calling /users/<user-id>/events if fails with the error:
Error: Operation: Create; Exception: [Status Code: NotFound; Reason: The requested user '<user-id>#72f988bf-86f1-41af-91ab-2d7cd011db47' is invalid.]
The GUID 72f988bf-86f1-41af-91ab-2d7cd011db47 apparently seems to be the TenantId of Microsoft according to this reference.
So what probably should be different is that I have to "tell" which tenant the user belongs to when registering the subscription in the case of a multi-tenant app registration. What I tried so far without success is the following: /users/<user-id>#<tenant-id>/events. The error message then is:
Status Code: NotFound; Reason: The requested user '<user-id>#<tenant-id>' is invalid.]
So the question is: How must I pass the tenant-id in the request so the correct user can be resolved?
Update
The error observed happens using the msal-client (#azure/msal-node) like this to obtain the graphAPI access token:
// Get a token using client credentials
const response =
await msalClient.acquireTokenByClientCredential({
scopes: ['https://graph.microsoft.com/.default']
});
// First param to callback is the error,
// Set to null in success case
done(null, response.accessToken)
Where the msalClient is instantiated like this:
const clientConfig: any = {
auth: {
clientId: clientId,
authority: authority,
clientSecret: configuration.getSyncServerClientSecret()
}
};
msalClient = new msal.ConfidentialClientApplication(
clientConfig
);
But requesting a token manually with Postman and then calling https://graph.microsoft.com/v1.0/subscriptions just works....
So the error must be with the usage of msalClient.