User key to toggle to and from RPN

[salvomic]

hi,
to toggle to and from RPN in Home we need about 4-5 clic (Shift-Settings, tap on Entry, tap on RPN or Textbook, ESC), it's boring to pass often from Textbook to RPN and viceversa.
I need a suitable user key to do that. Is it possible to use, i.e. Shift-User-Home, and how?

Better it would be to have a specific key on the Prime :-)

I'm having a great nostalgia for RPN also if I believe that on the Prime Textbook is more efficient for the things that I do on it...
However, if it is not so useful, it could be a little curiosity: how to?

thank you
salvo