Steps to claim :
Register on travel.mastercard.com
Make a phone call to Toll free: 0008 004 407 027 (put 3 zeroes as it is international toll free) and ask them for Marriott Silver Elite Upgrade Offer.
Fill the upgrade form and receive the upgrade within 3 days.
All the best enjoyy :)
Source...
In this wiki post I want to cover all banks GYFTR and related platform for voucher buying at one place.
S. No.Bank NameVoucher Buying Platform
1HDFChttps://www.gyftr.com/instantvouchers/
2SBI CChttps://www.gyftr.com/sbicard
3Kotakhttps://www.gyftr.com/kotak-instantvouchers/
4...