As already stated the context menu is mapped to a regular short press of the menu button (the button with an icon of three lines). As per the guide this remote in OSMC is not sending the keyboard “C” key.
Technical
All versions of the OSMC remote will map as follows if being used in OSMC running March 2020 update or newer…
Home = escape (key id="61467")
Info = leftbracket (key id="61480")
Up = up (key id="61568")
Down = down (key id="61569")
Left = left (key id="61570")
Right = right (key id="61571")
OK = return (key id="61453")
Back = browser_back (key id="61616")
Menu = rightbracket (key id="61481")
Play = play_pause (key id="61629")
Stop = stop (key id="61628")
Vol- = volume_down (key id="61624")
Vol+ = volume_up (key id="61625")