How do you make a store for tickets?

I having problem with the in-game store. How do you make so you can buy stuff for ticket in store(MarketplaceService)?

local ItemID =  
local Player = game.Players.LocalPlayer

script.Parent.MouseButton1Down:connect(function()
    Game:GetService("MarketplaceService"):PromptPurchase(Player, ItemID) 
end)

Here is my in-game store. How do i make so they can buy for tickets/Tix too?

Currently they can only buy for Robux.

Sorry my english.