How do I reply or send an email from a shared mailbox address, rather than my own email address?

Outlook (Desktop Version)

Open a new email

Go to Options tab > show Fields and select "from" to add that field to all new emails

From here, you can change the send address by clicking on the from field

Outlook (Online)

Go to Settings (top right near profile name)

Go to 'Compose and reply'

Under 'Message Format' tick the box 'Always show from'

Hit Save.

You can then change the address, by hitting the from field on a new message;