How do you make a projectile have rapid-fire mode when holding down a Key?

I would want the projectile to constantly shoot out by itself as long as the player hold the hotkey but I do not know how to script that.

I do know how to make it rapid-fire but the only thing i don't know is the holding down key function.Any help?

The script that enables the projectile:

--// Services \\--
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local UserInputService = game:GetService("UserInputService")

--// Varaibles \\--
local plr = game.Players.LocalPlayer
local Char = plr.Character or plr.CharacterAdded:Wait()
local Remote = ReplicatedStorage.FireBallEvent
local Mouse = plr:GetMouse()

--// Settings \\--
local Debounce = true
local Key = 'E'

UserInputService.InputBegan:Connect(function(Input, IsTyping)
 if IsTyping then return end
 local KeyPressed = Input.KeyCode
 if KeyPressed == Enum.KeyCode[Key] and Debounce and Char then
  Debounce = false
  Remote:FireServer(Mouse.Hit)
  wait(3)
  Debounce = true
 end
end)

The Set-up:

--// Services \\--
local ReplicatedStorage = game:GetService("ReplicatedStorage")

--// Variables \\--
local Remote = ReplicatedStorage.FireBallEvent

--// Settings \\--
local Damage = 10

Remote.OnServerEvent:Connect(function(plr, Mouse)
 local Char = plr.Character or plr.CharacterAdded:Wait()
 
 local Part = Instance.new('Part')
 Part.Shape = Enum.PartType.Ball
 Part.Anchored = false
 Part.CanCollide = false
 Part.Transparency = 0
 Part.Color = Color3.fromRGB(255,176,0)
 Part.Material = Enum.Material.Neon
 Part.Size = Vector3.new(1,1,1)
 Part.Name = 'FireBall'
 Part.CFrame = Char.HumanoidRootPart.CFrame
 Part.Parent = workspace
 
 local BodyVelocity = Instance.new("BodyVelocity")
 BodyVelocity.MaxForce = Vector3.new(math.huge, math.huge, math.huge)
 BodyVelocity.Velocity = Mouse.lookVector*30
 BodyVelocity.Parent = Part
 
 local Debounce = true
 Part.Touched:Connect(function(h)
  if h.Parent:FindFirstChild('Humanoid') and h.Parent.Name ~= plr.Name and Debounce then
   Debounce = false   
   local Enemy = h.Parent.Humanoid
   Enemy:TakeDamage(Damage)
   Part:Destroy()
   wait(1)
   Debounce = true
  end
 end)
end)

I already added a RemoteEvent in ReplicatedStorage.