how do you make a script only activate after purchasing a developer product?

you have to check if the player has the game pass

local id = -- your gamepass id here
local marketService = game:GetService("MarketplaceService")

game.Players.PlayerAdded:Connect(function(user)
    if marketService:UserOwnsGamepassAsync(user.UserId, id) then
        -- put whatever you want to happen here
    end
end)