code: private void configureCustomFields()
     {
        CustomFieldContainer customFields = m_projectFile.getCustomFields();
  
        // If the caller hasn't already supplied a value for this field
        if (m_activityIDField == null)
        {
           m_activityIDField = (TaskField) customFields.getFieldByAlias(FieldTypeClass.TASK, "Code");
           if (m_activityIDField == null)
           {
              m_activityIDField = TaskField.WBS;
           }
        }
  
        // If the caller hasn't already supplied a value for this field
        if (m_activityTypeField == null)
        {
           m_activityTypeField = (TaskField) customFields.getFieldByAlias(FieldTypeClass.TASK, "Activity Type");
        }
     }
docs: Find the fields in which the Activity ID and Activity Type are stored.