To my knowledge it's not possible to assign a hotkey to just a single bag -- the only one available is the hotkey to open/close all bags.
Of course, what you can do is drag that bag onto a hotbar, and then use the key assigned to that particular button of the hotbar to open it. If you move the bag around you'll have to drag it again, but it should do what you're looking for in a relatively simple way for you. It's not quite something you can just give out as a / command for others to use, however.
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