Assigning "Bixby" button function on S8+

Under system / buttons, how do I assign the “Bixby” button a function? Is that the “search button”? If so, it comes with “search assistant” as the preassigned function for a short press, but it doesn’t appear to do anything, even when I select a different function. (Also, what is the “search assistant” ?)

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Sorry, I necrobump this old post, but I’m in same situation.
The bixby button open “recent apps”. In settings I found some options to set “search button” that don’t seem have any effect.
While I’m satisfied with default bixby button behaviour, and I know I always can change it with magisks modules, I really would understand what is “search button” and if ‘bixby button’ is related to it.