Roblox MarketplaceService

local id = "" -- Change this to your id. Remove the quotes
local marketplaceService = game:GetService("MarketplaceService")
local player = game.Players.LocalPlayer

-- Make sure this is a button

script.Parent.MouseButton1Click:Connect(function()
        marketplaceService:PromptGamePassPurchase(player, id) -- Change this to PromptProductId if you are using a product
end