Box 1; User2 -
Billing Administrator -
Select Transfer billing ownership for the subscription that you want to transfer.
Enter the email address of a user who's a billing administrator of the account that will be the new owner for the subscription.
Box 2: Azure Account Center -
Azure Account Center can be used.
Reference:
https://docs.microsoft.com/en-us/azure/billing/billing-subscription-transfer#transfer-billing-ownership-of-an-azure-subscription