That field does exist: https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta/object_reference/sforce_api_objects_serviceappointment.htm
If you add a custom field, it will get ignored and will not add any value in the final saved appointment. However, if you set value in the assignment stage for this standard field, it will get saved to the appointment record, which flow will create.
[UPDATE]: If you do not care about anonymous booking, you can simply delete this assignment. No need to create a new field!