unable to map C-_ to undo in wsl ubuntu emacs

[Sivaram Neelakantan]

I've got Emacs-27.2 running in Emacs -nw and when I do C-_, I get RET
which I confirmed by C-h k "RET runs the command newline (found in
global-map)... compiled lisp function in 'simple.el'"

Now I try to reset to undo by doing M-x global-set-key and generating
the mapping by C-x ESC ESC which gives me

;; mapping C-_ to undo as it seems to be not recognized in some settings
(global-set-key "" 'undo)

This doesn't work and I can't figure out why C-_ is mapped to undo in
simple.el in one OS (win64) and RET in WSL ubuntu terminal Emacs simple.el.

How should I go about fixing this please?

sivaram
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