You should be able to put "PageDown" / "PageUp" or "ArrowDown" or "ArrowUp" but those keys also control the select window by default so it might be a bit awkward if using those to scroll both windows at the same time.
Viewing post in [CGMZ] Quest System for RPG Maker MZ comments
No problem, you can know what to type by going to a js key test website such as this (https://www.toptal.com/developers/keycode) and pressing the key you want. You want to use the "event.key" box (the top left box on the above website)