What scheduleId to use for non-primary calendar?

Question

Hi there,

I am calendar: getSchedule (https://learn.microsoft.com/en-us/graph/outlook-get-free-busy-schedule) to get busy time ranges for a delegated user.

By using Owner's email address as Schedule ID, I am able to read only the "primary" calendar.

How do I also read the other calendars in the same account?
I tried using calendarId, but that causes an error (The email address provided is not a valid SMTP address. with code 5035)
I also tried using Owner.Address, but also was in error (Active Directory operation did not succeed. Try again later. Code: ErrorADOperation)

What Schedule Id can I use?

Thank you.

https://graph.microsoft.com/v1.0/me/calendar/getschedule
{
    "Schedules": [
        "user@email.com"
    ],
    "StartTime": {
        "dateTime": "2024-04-23T00:00:00",
        "timeZone": "Asia/Singapore"
    },
    "EndTime": {
        "dateTime": "2024-04-23T23:59:59",
        "timeZone": "Asia/Singapore"
    },
    "availabilityViewInterval": "15"
}