Need help with activating a function while I'm hold a key down?

To run a function while holding a key you'll want to use the repeat loop and a simple boolean variable. Doing this, once you start holding the key, the loop will run until you stop holding the key. As seen here:

local holdingW = false
local userInputService = game:GetService("UserInputService")

userInputService.InputBegan:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.W then
        holdingW = true -- Being loop:

        repeat
            wait(0.2)
            print("Is holding W")
        until holdingW == false
    end
end)

userInputService.InputEnded:Connect(function(input)
    if input.KeyCode == Enum.KeyCode.W then
        holdingW = false -- End loop.
    end
end)

As you can see the loop will run until it detects a change within the variable. Even when made outside of the loop.