The mailbox flag that controls whether messages sent by a delegate get copied to the Sent Items of the original mailbox is not settable via Graph. We need to execute a PowerShell script to do this.
Set-Mailbox -Identity $mailbox -MessageCopyForSentAsEnabled $true
Our app's onboarding process, which requires setting this flag for a shared mailbox, would be greatly simplified if this could be done via the web, instead of having to provide a script and walk the customer through its manual execution.