You can do that if you have the latest beta installed, by adding this to the commnad:
KEYARGS "mmb:NEWTAB=nofocus"
The Favorites Bar command is a little special, due to having to translate the name of the favorites branch it displays, but the whole replacement command looks like this:
@ctx:="Favorites SHOWICONS USEQUALKEYS NOOPENINTABS AUTOCREATE BRANCH=""" + LanguageStr("FavoritesBar") + """ KEYARGS ""mmb:NEWTAB=nofocus"""