Answer
The pattern involves connecting a custom field back to its core entity via the Objkey column (which IFS normally hides from end users on the _CFT/_CFV structures), then building a Custom Event on the CFT (Custom Field Table) that passes the NEW:ROWKEY value when the record changes. A Validation-type workflow triggered by that event can then use an IFS Projection Call task to read the current record (filtered by the objkey passed in) and check whether the custom field's value is null, raising a validation error if the condition is met. This reuses the CFT/Objkey linkage rather than anything conditional-fields-specific, and is the closest current-Cloud equivalent to Apps9's Conditional Fields for this kind of cross-field mandatory-value validation.