How to use Send-as in OWA
To enable the “from” field, go to Options > Email > Layout > Message format and tick in the “Always show the from field” option.
Now you may create a new email, then click on your own email address at the “from” field and choose Remove:
Type the send-as email address, and OWA will offer the given user account:
Now you can send the email as the send-as user account.
The next time when you need to send from the send-as address, OWA will now offer it to you from the drop-down list: