I personally do that by adding this command to the file KeyEventTranslations.tr
file inside EventTranslations[{
(* Matching parentheses*)
Item[KeyEvent["9",Modifiers->{Command,Shift}],
FrontEndExecute[FrontEnd`NotebookApply[FrontEnd`InputNotebook[],
"(\[SelectionPlaceholder])", Placeholder]]]
However, this is not documented or recommended by Wolfram, and the functionality may not be maintained in future releases. The analogous command for adding []
or {}
or ""
used to work some years ago but has deteriorated over time. The file is in the directory ~/Library/Mathematica/SystemFiles/FrontEnd/TextResources/Macintosh/ on my system, and was copied there from the parent unmodified file in the main Mathematica directory.