You can write your topic however you want, but you need to answer these questions:
What do you want to achieve?
I want to make a gamepass for a certain killer to cost robux, meaning not to cost money and when it is purchased it can be used .
local MPS = game:GetService('MarketplaceService') -- gets the MarketPlaceService
local Player = game.Players.LocalPlayer
local ID = 0 -- id of your gamepass
MPS:PromptGamePassPurchase(Player,ID) -- this will prompt the purchase
And this can be used to check if the player owns the gamepass:
local MPS = game:GetService('MarketplaceService') -- gets the MarketPlaceService
local Player = game.Players.LocalPlayer
local ID = 0 -- id of your gamepass
if MPS:UserOwnsGamePassAsync(Player.UserId,ID) then
-- do code here
end