This site is being deprecated.
Please see the official X‑Plane Support page for help.
It is not possible to use X-Plane 10 USB Pro Key for X-Plane 11 but you can buy X-Plane 11 Professional-Use USB Key here.
I already searched the history before posting my question. The linked answer states
Unfortunately I don't think we plan to bring back the USB home use key for X-Plane 11 at this time.
I am not interested in the "USB home use key". I am interested in the USB Pro Key.
This site is no longer being actively maintained.