This one’s from an older forums post I came across:
This is a simple thing you can right in Studio. Just checkbox the ‘Audit’ option in the field preferences as shown below:
Or, if you want to make this change at the code level, you can add a vardef extension to the custom directory. So for example, to add a new field test_c in the Accounts module to the audited field list, add this into a new file named custom/Extension/modules/Accounts/Ext/Vardefs/test_c_audit.php
Afterwards, you’ll need to do a Quick Rebuild and Repair for the changes to take effect.