Synchronization Rule Editor

Hello everyone, I've an hybrid environment with a local on-premise Active Directory (contoso.com and externals.contoso.com) synced with Office 365 with Azure AD Connect.

I want to sync to Office 365 all users from domain contoso.com, and just the users with a specific keyword in the "Office" Active Directory field (physicalDeliveryOfficeName attribute) from domain externals.contoso.com.

I found this article https://www.lewisroberts.com/2015/09/06/filtering-objects-from-azure-active-directory/ that describes the rule creation for filtering users with the specific keyword in the Office field (without the domain selection).

How can I integrate the domain, for making rule valid just for externals.contoso.com domain?

Thanks to everyone!

Piero