Your OS key may be in the Digital Locker associated with the email address used to order the system. We won't have a record of your product key though, so that would be the best place to check. If the key isn't there, and I don't believe it would be loaded there, the only other thing I can think for you to try would be to use a Windows product key finder. I've only ever used Magical Jelly Bean, so I can't really make a recommendation on a specific software tool.

Dell Digital Locker: https://www.dell.com/support/software/us/en/04

Would you mind if you change your post title to reflect your question? This way if someone has a datastore issue, they know this post wouldn't be related.