Change key bindings of urxvt.tabbed

[Vladislav]

Hello!

How can I change key bindings of urxvt.tabbed
I prefer to use Super_L (keysym 0xffeb) instead of Shift
I know - the matter is urxvt::ShiftMask

I read this post 
http://superuser.com/questions/409900/urxvt-how-to-switch-among-tabs-like-other-emultaors
but this is not my case.

If we make grep ShiftMask /usr/lib/urxvt/urxvt.pm we can see "Mod1Mask, Mod2Mask". So, where 
can we find their definition?
Can I make for Mod2Mask = 0xffeb, for example? Or how can I make my own Mask?

Best regards