Hello!
The code is sent separately.
It seems like it will be sent on the 15th of September, which is when the Xbox service goes live for everyone, I think. And if someone redeems the voucher after the 15th, they will get the code 24hrs after.
From the terms and conditions:
"Customers redeeming their evoucher before 15 September 2020 will receive a code to redeem their 3 months of Xbox Game Pass Ultimate via email on 15 September. Customers using their evoucher after 15 September will receive an email with their code within 24 hours. Game Pass Ultimate must be redeemed at microsoft.com/redeem between 15 September and March 2, 2021."
Hope that helps