Hi,@Roger Roger
Thanks for posting your question in the Microsoft Q&A forum.
The command you provided is quite correct, it does assign Send-On-Behalf permissions to shared mailboxes.
If you want to revoke a user's Send-On-Behalf-Of privileges on a shared mailbox, you need to clear the members in the Send-On-Behalf of the shared mailbox first and then reassign the permissions.
1.Empty the members in Send-On-Behalf of the shared mailbox.
Set-Mailbox -Identity DL -GrantSendOnBehalfTo $null
2.Re-add member.
Set-Mailbox -Identity "smb@contoso.com" -GrantSendOnBehalfTo @{add="user1@contoso.com","user2@contoso.com"}
Note: Microsoft does not display the user's name directly but the GUID for the user's security. you can find out which user it is based on the GUID.
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