Similar to horse archers. Mounted crossbowmen existed for a time before advancements in firearms made them less useful than reiters or other similar cavalry with a brace of wheellock pistols. Reloads are harder than with conventional horse archers, but possible with either spanning mechanisms like the goat’s foot lever/cranequin or by using lighter crossbows you could span by hand.I'm in no way an expert in any of this, but I cannot fathom how your character is going to be mounted on a horse and shooting a crossbow.