You can definitely buy the tickets (no validation is required to do so, as you can also buy BC-discounted tickets for someone else), however, you cannot (a) earn points on those tickets (as you'd have to enter the BC number during the booking process) and you cannot (b) use yout not-yet-existent BC as a means of identification for your online ticket (you'd have to resort to an ID card or your credit card).

As far as I know, you may enter the day of first validity during the application process (at least that has been possible on written application forms, but I haven't applied for a BC since they imtroduced the subscription model).