How do you play b sharp on a bass clarinet?

B# is the same as C.

So low C would be all fingers down, low E key and the bottom thumb key (if your Bass Clarinet has a low C)

Middle C is thumb and 1st 3 fingers

Upper C is thumb, register key, all 6 fingers down plus the bottom C key (on either hand)

High C is thumb and register key

(Super C is thumb, register key and index fingers)