When you sign in to the Exchange admin center (EAC) in Microsoft Office 365 dedicated/ITAR, you see limited options to set the permissions of a shared mailbox. The option to individually manage the "Send on Behalf" permissions is available for user mailboxes. However, only the option to manage full mailbox permissions is available for shared mailboxes.
This issue occurs because the "Send on Behalf" permissions can't be managed in the EAC.
To work around this issue, add the "Send on Behalf" permissions by using Windows Remote PowerShell. To do this, follow the instructions on the following Microsoft TechNet website: