how to make entity jump while im riding it like a horse,

 

but im riding spider and i want to make it jump when i press space

how to set it up?

It is not that easy if you never used packets, but the way you do it is you handle InputEvent.KeyInputEvent and in it you check for:

Minecraft.getMinecraft.gameSettings.keyBindJump.isPressed()

 

and in case it is true, you send a packet to the server, upon handling, you set the player's ridingEntity's motionY:

player.ridingEntity.motionY += 1.0;

 

Which will make the ridden entity jump one block. Note that this will jump for whatever entity the player is riding. If you want it to only work for spider, check for:

player.ridingEntity instanceof EntitySpider