How to make your own particle game pass?

I am trying to create my own particle for players to buy a gamepass and get.

I have the premade particles from roblox working. Script:

local passId = 000000000
 
function isAuthenticated(player)
    return game:GetService("MarketplaceService"):PlayerOwnsAsset(player, passId)
end
 
game.Players.PlayerAdded:connect(function(plr)
    if isAuthenticated(plr) then
        plr.CharacterAdded:connect(function(char)
            local Trail1 = Instance.new("Sparkles")
            Trail1.Parent = char:WaitForChild("Left Leg")
            local Trail2 = Instance.new("Sparkles")
            Trail2.Parent = char:WaitForChild("Right Leg")
        end)
    end
end)

And I have created my custom particle and placed it in replicated storage, I used this script to test it for when I touch a block.

local rs = game:GetService("ReplicatedStorage")

function onTouch(hit)
	local ee = rs.ParticleEmitter:Clone()
	
	if hit.Parent.Torso:FindFirstChild("ParticleEmitter")then
		hit.Parent.Torso.ParticleEmitter:remove()
	end
	ee.Parent = hit.Parent.Torso
	end

script.Parent.Touched:connect(onTouch)

But no I am trying to make it so that you get my custom particle when you purchase a gamepass, this is what I have so far, and I am not sure what to do from now.

local passId = 000000000
 
function isAuthenticated(player)
    return game:GetService("MarketplaceService"):PlayerOwnsAsset(player, passId)
end
 
local particles = game.ReplicatedStorage.Particles
game.Players.PlayerAdded:connect(function(plr)
    if isAuthenticated(plr) then

Any help would be much apreciated thank you :)

Instead of using MarketPlaceService to check if a player has a gamepass you should use GamePassService. And check if they have the pass in the function itself. So then your first block of code would look like this

local passId = 000000000
local GamePassService = game:GetService('GamePassService')

	game.Players.PlayerAdded:connect(function(plr)
	if GamePassService:PlayerHasPass(plr,passId) then
	        plr.CharacterAdded:connect(function(char)
	            local Trail1 = Instance.new("Sparkles")
	            Trail1.Parent = char:WaitForChild("Left Leg")
	            local Trail2 = Instance.new("Sparkles")
	            Trail2.Parent = char:WaitForChild("Right Leg")
	        end)
	    end
	end)

Your second script looks fine

Your last script is it meant to suddenly end at 'then' because then it wont work.