JR Pass reserved seating

Firstly, I apologize if this question has been previously asked. My question is, am I able to reserve seats online ( with my voucher) prior to activating my rail pass, and if so how far in advance? I recently purchased 3 rail passes ( in the united states) and am awaiting my vouchers to come in the mail. I plan on activating and using the rail passes on the same day, May 27th, 2023, in japan. My question is, am I able to reserve my seats online when I receive my voucher in the mail ( in the unites states) prior to entering japan and activating my rail pass?