Do you want to add a shared calendar for a private channel or the private team?
If it is for a private channel, the channel calendar should work for you.
If it is for the private team, please try this url:
https://outlook.office.com/calendar/group/<domain name>/<team name>/view/month
If the team was created in Teams, please first set it to be visible in Outlook by running this cmdlet in Exchange Online Powershell:
Set-UnifiedGroup -identity <team name> -HiddenFromExchangeClientsEnabled:$false
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