I guess you mean the invitiation code (registration code) to setting up your Stadia account, right?
If your Mum bought the Stadia Premiere edition for you, she should have received an email to the primary email of her Google account. Normally this code is sent 2-3 days after the item is shipped. Let her check also the Spam, trash and advertise-folders, as the email might go in there instead of the inbox.
If you still can't find the code, your Mum can also contact Stadia support and request one if the hardware has arrived. She needs the order-number of the item (check in "orders" in store.google.com when logged in) and also the serial-number of the hardware.