How do I make a custom jump key (and unbind the original one)???

Hello there.

You must use ContextActionService. It is used for binding actions to keys.

You can first unbind the jump action by using ContextActionService:UnbindAction("jumpAction").

Then, you can use UserInputService to detect the input when the player jumped.

Example Code:

local UserInputService = game:GetService("UserInputService")
local ContextActionService = game:GetService("ContextActionService")

ContextActionService:UnbindAction("jumpAction")

UserInputService.InputBegan:Connect(function(input, gameProcessedEvent)
    if input.KeyCode == Enum.KeyCode.LeftControl and gameProcessedEvent then
        local character = game:GetService("Players").LocalPlayer.Character

        if character then
            if character.Humanoid.Health > 0 then
                character.Humanoid:ChangeState(Enum.HumanoidStateType.Jumping)
            end
        end
    end
end)

How it Works

You must first unbind the "jumpAction" action. Then, you must use the InputBegan event of UserInputService to detect when the client makes an action. Next, you must check if the KeyCode is the KeyCode you want and that the player isn't in chat. I used LeftControl as the jump key. Then, you must check if the character isn't nil and if the character's health is above 0. If it is, the humanoid will then jump.

Please accept this answer if it helped.

What I think that will work is use a body force or something like that and make it move the player upwards, then just set the jump power to 0. It is probably not the best way but it should work.