I've configured a custom field as such:
"vintrax:reviewer": { "name": "Reviewer", "type": "users", "search": true, "required": true },
However, whenever a user changes this field, this is shown as the example below in the Recent Activity feed:
User changed Reviewer from ["PHID-USER-wx672b7gtrpq6nflwbis"] to ["PHID-USER-ovbhgwiqrtq44asfv6mg"] on T506