If you are using Office 365 and have deployed the Signitic Add-in, you have the ability to set up signature delegations.
Delegation allows you to display a signature from a user other than the logged in user. It also allows you to have access to the signatures of your shared mailboxes on Outlook Windows.
To activate the delegation of a user, go to the profile of the user whose signature you wish to delegate (the shared mailbox for example) and then to the Settings tab:
Then, just type the emails of the users who will have access to this signature and don't forget to save!
On the Add-in side, thanks to the delegation, the right signature will be displayed directly when you write from the shared mailbox as well as when you open the Add-in.