Hello Bryan. You can use your Microsoft account balance to purchase Robux but only when you purchase through the Xbox platform. Your $5 gift card from Microsoft rewards went directly into your Microsoft balance (as opposed to just giving you a 25-digit code), which means it’s tied to your account and can only be used where Microsoft balance is accepted. Roblox’s website, mobile app, and PC app do not allow Microsoft balance as a payment method for Robux purchases. The only available route is to purchase through the Xbox Store; Robux can be purchased there using your Microsoft balance as long as you’ve signed in with the same account.
If you don’t have an Xbox console, you can still access the Xbox Store through the Xbox app on Windows or by visiting store.xbox.com in a browser. You just need to sign in with your Microsoft account, search for Robux, and complete the purchase with your balance.
Best regards,