Basically redeem a Microsoft gift card instead of the Rixty card and buy robux or BC through the ROBLOX windows store app using the credit you redeemed
On October 31st, 2021, Bing removed Rixty from the Redeem Shop. This method can still be used to buy Amazon Gift Cards, XBox cards, etc. You may even be able to use a Gift-Card trading site to redeem 80-90% of it for PayPal, and use that to get BC or Robux.
A new tutorial for how to redeem free Robux or Builders Club using Bing rewards and Rixty.