How can I bind a keyboard key to a function?

Hello helpers, I want to bind the key Left Ctrl to a function which reduces the players walkspeed.

Function =

local function ReduceSpeed()
	Player.WalkSpeed = 9
end

the Player variable is game.Players.LocalPlayer.Character.Humanoid. I've looked over the Roblox Wiki about ContextActionService, but I do not understand it at all. Someone please help! Thanks!

You can use the InputBegan event of UserInputService to detect whenever any key is pressed, and then check if the key is left control, like so:

game:GetService("UserInputService").InputBegan:Connect(function(iobj, gp)
	if gp then --this variable is true if they are i.e. typing the text into a textbox
		return --so we abort if this is the case
	end
	if iobj.KeyCode == Enum.KeyCode.LeftControl then
		ReduceSpeed()
	end
end)