Custom field to list users

How to create new CF to list RT user?
I need to create special type of ticket assignment for tracking purpose only (not related to ACLs nor Actions at the moment).

You should start from here: https://docs.bestpractical.com/rt/4.4.3/extending/external_custom_fields.html

Check for CustomFieldValueSources or also check this example: https://github.com/tbrumm/rt-extension-assettracker/blob/3.0-trunk/lib/RT/CustomFieldValues/ATServers.pm

Torsten