Put it wherever you find it comfortable. Unlike a controller, you have loads of keys to play with I put Secondary/Aim/Zoom on the Space bar and hit it with my thumb. That way, I can be zoomed in leaving my fingers free for movement keys.
Over the years, I’ve moved away from WASD completely, so my standard loadout is a bit bizarre:
number keys - weapon slots (for Borderlands games, I bind grenade to “5”)
qwert - sprint, crouch, forward, back, jump (I bind “forward” to a key, as well, if the game options allow)
asdfg - reload, skill, left, right, use
space - secondary/zoom
mouse l/m/r - primary, melee, forward
I find this keeps my fingers side-by-side rather than scrunched up around WASD. Mouse + asdfg cover 80%+ of gameplay.
I use this basic set-up for every shooter with minor variations. But this is completely personal. It depends on the size of your hands, keyboard layout and the way your brain works Experiment. You can always hit “reset” and start again. Good luck.