Yeah. Press the key without modifier: get underscore '_'. Press the key with shift: get '-'. Return to default with: xmodmap -e 'keycode 20 = minus underscore

Yeah. Press the key without modifier: get underscore '_'. Press the key with shift: get '-'. Return to default with: xmodmap -e 'keycode 20 = minus underscore minus underscore' (This is for a qwerty layout)