It doesn't work as that's one of the default
gnome-shell shortcuts. So
gnome-shell actually grabs it even if you set it to execute some custom action. You'll have to remove it first from the default shortcuts list - easiest way is via terminal with
gsettings set org.gnome.shell.keybindings toggle-message-tray "['<Super>v']"
alternatively, if you prefer doing it via gui, fire up
in the left pane, navigate to
keybindings then in the right pane right click on
toggle-message-tray and select
a new window will pop up: toggle
Use default value to OFF:
then edit the custom value, removing
, '<Super>m' so that it reads:
['<Super>v'] then hit Apply.
Restart your shell with Alt+F2 then r then Enter (or log out & log in). You should then be able to use your custom Super+M shortcut.