For Microsoft 365 shared mailboxes you need to contact IT to set the option.


otherwise, for the exchange webapp follow these steps:

 

  1. Log in to OWA of a mailbox you have send as/full access or send on behalf of permissions.

  2. Navigate to Options in the upper-right corner> See all options > Settings tab > Sent Items tab.

  4. Messages sent as this mailbox can be saved in the Sent Items folder of the:
    • From mailbox;
    • Sender and From mailbox;
    • Sender mailbox.
   Select desired option from the first drop-down list > click Save.

   The same options are available for messages sent on behalf of this mailbox.