Sorry, I haven't been ignoring this question, I've just been very busy lately.
There isn't currently a way to select the result item from the preview window. The window itself is non-interactable, so there's no chance for the player to cursor over it.
There are three ways I could try to tackle this request, and I'm not sure which, if any, is better.
The first way would be to have the help window switch between the description of the ingredient (or the craft button) you currently have selected, and the result item's help text, if it exists, after a set amount of time.
The second way would be to have a hotkey that the player could push to switch the help windows text to the description of the result item, and back, probably shift or control.
The third way would be to have an extra 'item' in the bottom of the ingredient window, on the left side, next to the craft button, that when selected, shows the help description of the result item.
There are obvious flaws in all of the above though. The first case is probably the least messy, but there isn't a good way to indicate what item the help text is for, if the window is just automatically swapping around every five seconds or whatever.
The second case might work fine, but there's no real way to inform the player that they could do that (outside of that button help window that visustella has in MZ), so they could go the entire game without knowing about it at all, or by accidentally pressing it and never seeing the help description of an ingredient again!
The third case causes a visual disruption to the ingredient list window. The craft button is smaller than the items in the list, so there's room for it on the left side, but I'm not sure what you could name it where it wouldn't stand out as something obviously put there as an afterthought.