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You cannot manually link a Roblox account to a Microsoft account on a website. The “link” only happens inside the Roblox app on Xbox or the Microsoft Store version of Roblox.
To use Microsoft balance for Robux:
- Install Roblox from the Microsoft Store (on PC)
- Sign in to your Roblox account inside that app
- Buy Robux there, it will attach to the account you’re logged into
If you buy from xbox.store.com without ever signing into Roblox via the Xbox/Store app, the Robux may not show up.
If that already happened, try signing into the Roblox Microsoft Store app with the same Microsoft account, or contact Microsoft support for a refund if it doesn’t appear.
Hope you'll get this sorted out!