I think a macro would work; something like the following (warning: untested)
EDIT: I don’t know why I read “shift” as “control”
enum macros {
// this could be whatever you want, just calling it SAM for Shift-Alt-Minus
MACRO_SAM,
};
const macro_t *macroAction(uint8_t macroIndex, uint8_t keyState) {
if (macroIndex == MACRO_SAM) {
return MACRODOWN(D(LeftShift), D(LeftAlt), T(Minus), U(LeftAlt), U(LeftShift));
}
return MACRO_NONE;
}
// Then put M(MACRO_SAM) in your keymap at the appropriate place