Issue setting up signatures in group mailboxes

I'm having issues with setting up signatures in group mailboxes in office 365. In previous versions of outlook, from within a shared inbox, users could create a new email and it would automatically be sent from that mailbox, along with a signature created for that mailbox. I'm finding that I can't get this to work with outlook 365. When a user creates a new email, the 'from' field defaults to there personal email. They can change this in the 'from' field, but the default signature is always there own. I've been into the signature settings and it wont allow me to set a default signature for any other mailbox except the primary one. This is incredibly frustrating because it seems like it should be such a simple thing to set up but after spending hours on this I'm stumped.