As an alternative to the procedure provided here, Microsoft provides a video on how to send from a shared mailbox in Outlook 2013 and Outlook 2016.
- Compose New Email.
- Look for the the From drop down field. If it is not there, go to the Options tab and select From from Show Fields to enable it.
- Click the From drop-down menu and select Other E-mail Address…
- Type the from address you want to add and click the OK button.
- To compose a message, click the From drop-down menu and select the address you have permission to.