I've tried to get the Calendar ID using this command in Powershell,
Get-CalendarProcessing -Identity "Meeting Room 1" | FL
It returns the below about IDs, but none of these are what's needed for FLOW as I tried them and they error. But I'm thinking there has to be a command to pull the ID from a given calendar in your own 0365 domain. The runspaceid changes every session you run, so I know it can't be that. The above command pulls a whole list of information in addition to what I put below, but no Mailbox ID. So still looking for the Calendar ID as everyone else here. Add my name to the list.
RunspaceId : faccc619-787e-4fa0-85a0-df2b172a12e2
MailboxOwnerId : Meeting Room 1
Identity : Meeting Room 1