Novice here. Looking for guidance on a script that would help me find out who the last person that modified a specific field.  

Example:

We have a custom module and I want to capture which person manually updates a specific field.  Whenever 'Custom Field XYZ' is edited/updated, input the user updating this field into 'Last Updated By' Custom field. I cannot use the existing Modified By field as we have found from Zoho CRM support that because we have functions updating fields, it'll always show super admin as the last modified by.

Appreciate your help