I'm not with Laminar Research, just a fellow simmer in the UK, but if you have purchased a valid digital download version of X-Plane 11 from LR, then you would have been sent an email with your Product Key as per the following URL:- https://www.x-plane.com/kb/digital-download-install/
1. If you have mislaid your email, then contact LR at @ https://www.x-plane.com/support/key-recovery/
2. If you have purchased a DVD copy of X-Plane 11 from LR, then I do not believe that there is a Product Key as such, you just need to keep your Disc 1 in the DVD ROM tray at all times.
3. If you purchased you copy of X-Plane 11 from another 3rd party supplier, then you will need to contact them directly for support.
I noticed that you asked a similar question about a lost Product Key in May 2018 on this Q&A forum and the advice you were given then, if you have a valid copy of X-Plane 11 purchased from LR, still stands!