We have customized the Appointment form in CRM with fields which are mandatory (both in the technical aspect as well as from the business perspective).

When we create a new Appointment in Outlook directly from the calendar, on the Appointment form, we click the Dynamics365 App. Here the only real option is to establish the tracking of the Appointment but there is no way to actually show and maintain the CRM fields (mandatory or not).

Strangely enough, when you create a new appointment out of an existing email or appointment, then you have also CRM fields available (via "+" button).

Suggestion: Make the CRM fields available and show them in the Dynamics365 App when creating a completely new Appointment from the Outlook calendar > as soon as user decided to track appointment in CRM.