You don’t necessarily need to focus on anything, just focus on relaxing. That’s the goal of the breathing exercises before void meditation. Relaxing so the mind calms down and there’s less thoughts so you can be more in control of your mind and have less anxiety.
Less anxiety= less racing/uncontrollable thoughts.
You can do the yogic complete breath or the sun/moon alternate nostril breathing for this^.
Both are in the breathing exercises section on the JoS.