100kV is serious business. Definitely don’t hook 100kV directly to your Mooshimeter. I know you know this but I just want to be clear for anybody who finds this thread in the future.
Any appropriately rated divider probe that brings 100kV down to less than 600V would work. 1000:1 divider probes are pretty standard. Like this kind.
Most divider probes rely on the meter having a specific input impedance. When measuring in the 600V range, the voltage input of the Mooshimeter has a 10MΩ +-1% input impedance. When measuring in the 60V range, the input impedance is 10.1MΩ +-1%.
Hope this helps – stay safe!