Click New Email.
If you don't see the From field at the top of your message, click on the Options menu and click From in the section labeled Show Fields.
(The From field appears above the To field as pictured below)
Click From in the message, and change to the shared email address. If you don't see your shared email address, choose Other email address, then click From... and select the email address you want from the list.
Finish typing your message and then choose Send.
From now on when you want to send a message from your shared mailbox, the address will be available in your From drop down list.
Whenever you send a message from your shared mailbox, your recipients will only see the shared email address in the message.