How do I make my clothing GUI find an asset ID from a catalog ID?

How do I make my clothing GUI find an asset ID from a catalog ID? Normally, I would just make it do - 1 after the ID, but with how many things are uploaded, that's not really an option anymore. Here's my code:

button = script.Parent.Parent.shirtadd
shirtid = script.Parent.Parent.Parent.Shirt
plr = game.Players.LocalPlayer
char = plr.Character


button.MouseButton1Down:connect(function()
	for i,v in pairs (char:children()) do
		if v:IsA("Shirt") then
			v:Destroy()
		end
	end
	local shirt = Instance.new("Shirt", char)
	shirt.ShirtTemplate = "http://www.roblox.com/asset/?id=".. shirtid.Text
end)

just go into the characters properties and change game.Workspace.player.Shirt.templateid (or whatever) = [your shirt id] and there shirt will change, (P.S. i dont think its called template id but write whatever its called)

Idk if this helps but this is the clothing gui script I use in my game (works great)

script: https://pastebin.com/raw/172GcKpb

I have changed the script to work with your gui, well Ive tried at least