SOAP API Contacts - convert code from Custom Field to Obj Designer Package fields

Summary

how do we convert our C# code to update Contact package fields instead of Custom Fields

Content

We've been updated Contact custom fields successfully for years, however in OSvC we are converting these CFs to fields in the Object Designer in a Contacts package "CO".

What is the simplest way to convert our C# code to update Contact package fields instead of custom fields?

(the package fields will have the same name). Code snippet is the skeleton of the c# that is currently used to update on CF "dept".

On attached PNG, there is the new "Dept" field in Object Designer package.