I think i got an email from carmax with this info, and also paperwork when i bought my car. I would check your records for this. If that doesn't help, call them or Google it.
If you have the card that came with the warranty paperwork, which has the warranty provider's number as well as your contract id, use that info. If you don't have a contract number the provider may not be able to help you. If you take your car to Carmax for warranty work, they should take care of the claim for you.
Check the website it should be there or call main number and ask
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