Relevant Products: Exclaimer Cloud - Signatures for Office 365 | Exclaimer Cloud - Signatures for G Suite
You wish to apply signatures to emails sent as shared mailboxes.
Server-side signatures can be applied to emails sent as shared mailboxes in the same way as you would do for a standard user:
How to apply a server-side signature
In order for the data for shared mailboxes to be successfully synchronized with Exclaimer Cloud, a shared mailbox must not be disabled in Active Directory or have Sign-in blocked in Office 365. Microsoft have confirmed that a license will not need to be applied to a shared mailbox providing that the account is not logged into. For more info see: Signatures are not applied to emails from shared mailboxes