How to change clothes every time player spawns?

Hi, i've been trying to make it so that every time a character spawns, their pants turn into this: https://www.roblox.com/catalog/3327167553/Green-Pajamas. However, I've been trying lots of different things but i'm way too inexperienced with Roblox Studio/Lua scripting to know what i'm doing (This is my first time attempting to script on Roblox/Lua ever). Where should I put my code to make this happen? What should my code be? What kind of script (local, regular, etc) do I need to use to make this happen?

My main attempt was this:

local function givePajamas(player)
    local character = player.Character
    if character then 
    	-- look for pants
    	local pants = character:FindFirstChildOfClass("Pants")
    	-- create pants if they don't exist
    	if not pants then 
    		pants = Instance.new("Pants", character)
     	end
    	-- reset pants content id
    	pants.PantsTemplate = "https://www.roblox.com/catalog/3327167553/Green-Pajamas"
    end
end

givePajamas()

I put this code as a local script inside of StarterCharacterScripts. I tried to use the ROBLOX Dev API Reference on their website to help me out (which is where I got the bulk of this code from) but nothing has seemed to work.

Appreciate any help, thank you very much!