The first auditor will not appoint in AGM as per Companies Act, 2056.
It is required for both. The Provision of filing of Form-23B withe ROC is apply to both, first as well as subsequent appointment.
When you file your first Form-23AC they demand SRN of Form-23B, in that case you will give the details of your first auditor.
So, as per requirement of Form-23AC, form 23B is mandatory for all appointments.
In a new Form -23AC point no. 10(II) (e) required to fill SRN of Form 23B which is in mandatory in nature. Please checkpoint no 10 of the file attached herewith.