Can i install F-Secure KEY on a windows 10 phone?
No. There is no support for Windows 10 Mobile.
Don‘t expect developers are putting much effort in supporting Windows 10 Mobile (no new Windows Mobile devices and very low market share).
I did my reply (under spoiler) too much long... possible to ignore this reply -> because previous two are good ones!
As noted with next resources:
Windows Phone (8/10; or even for previous realizations as Windows Mobile) is not supported platform for F-Secure Key on current days;
Under community was some requests for support it (Windows Phone 8 and current Windows 10 mobile-platform). Some examples of topics (where possible to read official statements or feelings from other community users):
Also possible to 'vote' for next feature-request:
With less words: F-Secure Key team looking forward (whatever it mean) and feature request with 'Future Consideration'-state.
This is also (maybe) valid for platforms like Linux, Jolla/Sailfish, Google ChromeOS;
OK, ill get the picture:-(