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