While you can buy Silver from the Xbox website, you would need to find a way to sign into the specific platform in order to have it added - Xbox (PC) and Xbox (Console) are separate platforms. What to do is send in a missing silver ticket via 'submit request' at the bottom of the following page: https://help.bungie.net/hc/en-us/articles/360049497031-Contact-Form-Missing-Silver Bungie might be able to offer some workarounds they could do on their end (ie purchase it -> share transaction info -> they clear the balance). Ultimately though, the fix will almost guaranteed include you having to sign in on an Xbox so the balance updates via the transaction rather than workaround.