For sending a mail on belhalf of some one throught OWA , you need to have full access on the mailbox.  There is no option to just add from: and add email address and send it. You have to open the required mailbox and send the email. 

1. You can select the drop-down box next to your name in the main menu bar, then enter the name of another mailbox to open. This will open the mailbox on the new window.