How would you use the processreceipt callback in another script?

So I got a ProcessReceipt script (server) from the roblox dev page, put it in my game and now I need to know how to use it in another script (local)

here's the code so far:

local player = game.Players.LocalPlayer
local ms = game:GetService("MarketplaceService")
local product = 797012396

function activate()
ms:PromptProductPurchase(player, product)

game.ServerStorage.ESItems["AA12"]:Clone().Parent = player:WaitForChild("Backpack")
game.ServerStorage.ESItems["AA12"]:Clone().Parent = player:WaitForChild("StarterGear")

end

script.Parent.MouseButton1Click:Connect(activate)