On the previous release of CA Service Desk, it is done by adding the custom filed to a *.mod file, such as cr_site.mod file.
On CA Service Desk r12.7 and r12.9, it is set through Web Screen Painter.
This document explains how to generate an Activity Log when the custom field is updated on r12.7 and later
In this example, the Activity Log "Field Update" will be logged when the custom field "zresolved" is update.
OBJECT cr { ATTRIBUTES Call_Req { zresolved INTEGER { UI_INFO "AUDITLOG";}; }; };