does this work

-- setup local variables
local MarketplaceService = Game:GetService("MarketplaceService")
local ds = game:GetService("DataStoreService"):GetDataStore("PurchaseHistory")
local productId = 19372077

-- define function that will be called when purchase finished
MarketplaceService.ProcessReceipt = function(receiptInfo) 

	-- find the player based on the PlayerId in receiptInfo
	for i, player in ipairs(game.Players:GetChildren()) do
		if player.userId == receiptInfo.PlayerId then
			
			
			-- check which product was purchased
			if receiptInfo.ProductId == productId then
				
				-- handle purchase. In this case we are healing the player.
				player.Character.Humanoid.Health = 100
				-- more feedback for the player.
				game.Workspace.DisplayScreen.SurfaceGui.TextBox.Text = player.Name .. " has purchased a healing potion."
			end
		end
	end	
	
	-- record the transaction in a Data Store
	local playerProductKey = "player_" .. receiptInfo.PlayerId .. "_purchase_" .. receiptInfo.PurchaseId
	ds:IncrementAsync(playerProductKey, 1)	
	
	-- tell ROBLOX that we have successfully handled the transaction
	return Enum.ProductPurchaseDecision.PurchaseGranted		
end

is a script in a the workspace

and this one is in the textbox

--[[This script goes inside of a TextBox which should be inside of a ScreenGui]]
productId = 19372077 -- Replace 123123123 with the id of your developer product.

local MarketplaceService = Game:GetService("MarketplaceService")
function UsernameFromID(ID)
	if type(ID) ~= "number" then
	return
	end
	local sets = game:service("InsertService"):GetUserSets(ID)
	for k, v in next, sets do
		if v.Name == "My Models" then
			return v.CreatorName
		end
	end
end
function giveRewards(player)
	-- do stuff you want to do here, using the variable player for the player that clicked it. To access the character, do player.Character.
	return Enum.ProductPurcaseDecision.PurchaseGranted
end
MarketplaceService.ProcessReceipt = function(receiptInfo)
	giveRewards(UsernameFromID(receiptInfo.PlayerId))
end
script.Parent.MouseButton1Down:connect(function()
	Game:GetService("MarketplaceService"):PromptProductPurchase(script.Parent.Parent.Parent.Parent, productId)
end)
AnonymousEdited