How would I use the MouseButton1Down function in this instance?

I'm working on a boss script and I want a gui to pop up saying would you like to join with a decline and accept button. I have the script finished and will list most of it below. Please tell me how I would use it like that.

-- Variables --

local player = game:GetService("Players").LocalPlayer

local ReplicatedStorage = game:GetService("ReplicatedStorage")

local ServerStorage = game:GetService("ServerStorage")

local BossArenas = ServerStorage:WaitForChild("BossArenaMaps")

local Status = ReplicatedStorage:WaitForChild("BossInformation").Values.Status

local Remaining = ReplicatedStorage:WaitForChild("BossInformation").Values.PlayersRemaining

local timeleft = ReplicatedStorage:WaitForChild("BossInformation").Values.Time

local TimeLength = ReplicatedStorage.BossInformation.Values.Time -- Time for each game

local invalid = "Cannot Find SpawnPoints"

-- Start --

while true do
	
	repeat wait(1) until game.Players.NumPlayers >= 1 
	
	-- UI --
	--[[local battlejoin = player.PlayerGui.battlejoin
	battlejoin.Enabled = true
	
	battlejoin.backround.accept.MouseButton1Down:Connect(function()
		
	end)--]]
	
	Status.Value = "Creating Boss.."
	
	wait(.5)
	
	Status.Value = "Creating Game.."
	
	wait(10)
	
	local plrs = {}
	
	for i, player in pairs(game.Players:GetPlayers()) do
		if player then
			table.insert(plrs,player) -- add players into table
		end
	end
	
	wait(3)
	
	local Arenas = BossArenas:GetChildren()
	
	local ArenaChosen = Arenas[math.random(1,#Arenas)]
	
	Status.Value = ArenaChosen.Name.." has appeared"
	
	local CloneArena = ArenaChosen:Clone()
	CloneArena.Parent = workspace
	
	local battlegui = player.PlayerGui.inbattle
	battlegui.Enabled = true
	
	-- Teleporting Players --
	
	local SpawnPoints = CloneArena:FindFirstChild("SpawnPoints")
	
	if not SpawnPoints then
		print(invalid)
	end
	
	local SpawnPointsAvailable = SpawnPoints:GetChildren()
	
	for i, player in pairs(plrs) do
		if player then
			character = player.Character
			
			if character then				
				
				 	-- If theres a character teleport them to arena
				character:FindFirstChild("HumanoidRootPart").CFrame = SpawnPointsAvailable[1].CFrame
				table.remove(SpawnPointsAvailable,1)
				
				local GameTag = Instance.new("BoolValue")
				GameTag.Name = "GameTag"
				GameTag.Parent = player.Character
			else
				-- If there is not a character remove them from table
				if not player then
					table.remove(plrs,i)
				end
			end
			
		end
	end
	
	
	Status.Value("Game Starting...")
	wait(2)
	
	for i = TimeLength,0,-1 do
		
		for x, player in pairs(plrs) do
			if player then
				character = player.Character
				
				if not character then
					-- the player left the game
				else
					if character:FindFirstChild("GameTag") then
						-- the player is still here
						print(player.Name.." is still in game")
						
					else
						
						-- player left game
						table.remove(plrs.x)
						print(player.Name.." is removed.")
					end
				end
			else
				table.remove(plrs,x)
				print(player.Name.." is removed.")
			end
		end
		
		timeleft.Value = "Time Left: "..i
		Remaining.Value = "Players Remaining "..#plrs
		
		if #plrs == 1 then
			-- Plrs
			Status.Value = "this person is coool"
		elseif #plrs == 0 then
			Status.Value = "nobody is alive"
			
		elseif i == 0 then
			timeleft.Value = "TIME IS OUT!!"
			break
		end
		
		wait(1)	
	end
	print("game ended")
	
	for i, player in pairs(game.Players:GetPlayers()) do
		character = player.Character
		
		if not character then
			-- ignore em
		else
			if character:FindFirstChild("GameTag") then
				character.GameTag:Destroy()
			end
			
			if player.PlayerGui:FindFirstChild("inbattle") then
				player.PlayerGui.inbattle.Enabled = false
			end
		end
		
		player:LoadCharacter()
	end
	
	CloneArena:Destroy()
	
	-- game end	
end

If the gui is on the boss, you could use a BilloardGui and make it so when the click on a button it does whatever needs done. If you want a notification gui on screen you could use a touched event on a part so when they touch it it fires the client to show the gui and then it does the stuff you need.