I'm not with Laminar Research, just a fellow simmer in the UK, but where did you purchase your copy of X-Plane 11 from? Was it from Laminar Research or from Steam?
If it was purchased from Laminar Research, then you should have received an email from them 'Thanking You' for your purchase as well as providing you with your 24 character X-Plane 11 Product Key XXXX-XXXX-XXXX-XXXX-XXXX-XXXX. If you didn't receive this email, then you will have to contact LR directly.
For lost, disabled, or stolen digital download product keys purchased from their site, please e-mail email@example.com.
For a copy of X-Plane purchased at their website please e-mail firstname.lastname@example.org or call 913-269-0976 for support.
For questions about a copy of X-Plane purchased on Steam, please e-mail email@example.com.