Turn off recurring billing then you can stack. I've stacked twice through 2020
When I started I had gamepass until 3/25/2019, and was able to extend it further. Make sure you have recurring billing turned off by going to microsoft.com > my account > view my products > xbox game pass > manage > "Has recurring billing, $59.94 every 6 months" > change > turn off recurring billing.
It's got a bunch if steps, and every time I stack another 6 months, it turns on recurring billing again, so you have to go back in and turn it off.
This is for XBOX Game pass which is normally $10/month.
YMMV. Some people are reporting they are not seeing the deal even with turning recurring billing off. Good luck!
here is the official tweet: https://twitter.com/Xbox/status/1...48416?s=19