Hi クリス, I hope your doing well despite the problem you are experiencing now and I'm sorry for that. For that let me try to assist you.
You will need to log-in to your Microsoft Account to the Microsoft Store website, then navigate to the Redeem a gift card or code section. From there Enter the code you received for your Robux then you can Confirm the redemption. Your balance will be updated through your Robux. If you do not see it, try logging out and in again.
Incase you encounter issues, you can check through Microsoft Support page as they have more detailed troubleshooting incase a problem occurs.
I hope this helps. Please let me know if I didn't address your problem. Don't hesitate to ask for further assistance.
Regards,
Arnel