Ok so I made required, then also set this field to be a dropdownlist that is populated by a SQL Query on the CMS_User table.
However I've realized that with close to 4000 rows in CMS_User, a dropdownlist is not user-friendly.
I've reverted back to using Kentico's User Selector form control. However this form control saves the UserID value to the row.
What I really need now is to create an accompanying computed field, that computes a value based on the value of the User Control field. Is it possible to configure a calculated/computed field to derive the FullName value for a given UserID value from another field?