Key commands are added to tool tips automatically for buttons in Dorico’s user interface (or at least for most of them; now that we are using both the older Qt Widgets APIs and the newer Qt Quick/QML APIs, it’s not universal, since it’s only implemented for our Qt Widgets buttons), but only where the underlying command represented by the button has a key command associated. In the case of the buttons in the right-hand notations toolbox in Write mode, those buttons hide and show their corresponding panels, and they don’t have shortcuts by default. Popovers are totally separate commands. If you were to assign a shortcut to the Write > Panels entry for a given panel, then you would see the shortcut in the corresponding button in the right-hand panel.
So, as Leo said in his original reply in this thread, you will get the shortcut in the tool tip if the button corresponds to a command with a shortcut. If it doesn’t, you won’t.