Update: March 31, 2021
Custom fields are now supported for all supported record types with the exception of events.
Calendar Events Only
If you are working with calendar events your NetSuite admin may have provided you with a default even form that has required custom fields. Since there are no similar fields in Outlook even creation will fail. As a workaround ask your admin to create a new form without marking the custom fields as required. This form can be provisioned to a new role (your current role can be cloned). Use this new role to login to NetSuite via CloudExtend. Since the fields are no longer required you will be able to save events.