How do I do the thing below?

For future reference, try and attempt it yourself before asking for help.

local Prompt = script.Parent
local MPS = game:GetService('MarketplaceService')
local gamepassID = 0 -- change to your gamepass

Prompt.Triggered:Connect(function(Player)
	if MPS:UserOwnsGamepassAsync(Player.UserId, gamepassID)
		-- do stuff
	else
		MPS:PromptGamePassPurchase(Player, gamepassID)
	end -- gamepass own check
end) -- onTriggered

Sources:

MarketplaceService

ProximityPrompt