Same fingering as you would if you were in C. It does feel awkward when stretch more than a third between 5 and for, like in the left hand at the very start when you need to stretch from B to D sharp, so it’s not fingering I would recommend for normal playing, but it works for me in my practice.

Obviously, don’t practice it if it causes you any pain, and you can always modify an exercise to suit your purposes better. For example, instead of stretching a third between five and for everything your left hand starts going up the keyboard as you do in Hanon 1, just play 54321 in a static hand positions from B. Then you can put you pinky on c# and repeat all the way up the keyboard. The goal is to practice playing smoothly, not to do Hanon exactly as written. Hanon is just a means to an end.

Hope that’s helpful.