Field activation alternative for the Resource's ManagerID field

Summary:

Our client needs to add a search field to the options of adding Resources to Activities in CX. Currently, there is a Custom Field called "Executive Code", which represents an internal identification of the institution, basically you want to search for Resources through the "Executive Code" of the Manager, since the Manager is a resource as well.
This new field would be called "Manager Code", a new Custom Field which needs to be filled with information.
The solution I was working on consisted of a script that was executed through a FieldTrigger of the ManagerID field. This field, ManagerID, changes each time the Manager is updated to the Resource. However, since the Resources are part of an Organization and it is this Organization that owns the Manager, the Field Trigger is not executed.