Don't want to write this by hand? GodCraft sets minecraft:horse.jump_strength for you with a switch.Get GodCraftOverview
minecraft:horse.jump_strength is an entity component: it goes in the components object of a custom mob, or in one of its component groups. Sets how high a horse-like mount can jump when the rider charges a jump.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:horse.jump_strength": {}
}