How do i transfer a local textbox string to a server string?

-- local script
local repStorage = game:GetService("ReplicatedStorage")
local Broadcast = repStorage:WaitForChild("Broadcast")

script.Parent.MouseButton1Click:Connect(function(player)
	
local MessageBox = script.Parent.Parent.MessageBox
local CharacterValue = script.Parent.Parent.CharacterLabel.Characters.Value
local CharactersLeftValue = script.Parent.Parent.CharacterLabel.CharactersLeft.Value
local MaxCharactersValue = script.Parent.Parent.CharacterLabel.MaxCharacters.Value

	Broadcast:FireServer(player, MessageBox, CharactersLeftValue, CharacterValue, MaxCharactersValue)
end)

and

game.ReplicatedStorage.Broadcast.OnServerEvent:Connect(function(player, MessageBox, CharactersLeftValue, CharacterValue, MaxCharactersValue)
	for i,v in pairs(game.Players:GetPlayers()) do
	local InstanceGui = v.PlayerGui.InstanceGui
	local MessageFrame = Instance.new("Frame", InstanceGui)
 	MessageFrame.Name = "MessageFrame"..math.random(1,1000)
	MessageFrame.Size = UDim2.new(0.2, 0,0.2, 0)
	MessageFrame.Position = UDim2.new(0.029, 0,0.703, 0)
	MessageFrame.BackgroundColor3 = Color3.fromRGB(102,102,102)
	MessageFrame.BorderSizePixel = 3
	MessageFrame.BackgroundTransparency = 0
	local AspectRatio = Instance.new("UIAspectRatioConstraint", MessageFrame)
	AspectRatio.AspectRatio = 1.5
	local Title = Instance.new("TextLabel", MessageFrame)
	Title.Name = "Title"
	Title.Size = UDim2.new(1, 0,0.2, 0)
	Title.Position = UDim2.new(0, 0,0, 0)
	Title.BorderSizePixel = 3
	Title.Font = Enum.Font.GothamBold
	Title.Text = "Server Broadcast"
	Title.TextColor3 = Color3.fromRGB(255,255,255)
	Title.BackgroundColor3 = Color3.fromRGB(102,102,102)
	Title.TextScaled = true
	local Text = Instance.new("TextLabel", MessageFrame)
	Text.Name = "Text"
	Text.Size = UDim2.new(1, 0,0.798, 0)
	Text.Position = UDim2.new(0, 0,0.2, 0)
	Text.BorderSizePixel = 0
	Text.Font = Enum.Font.GothamBold
	Text.Text = tostring(MessageBox.Text) -- Mainly this part
	Text.TextColor3 = Color3.fromRGB(255,255,255)
	Text.BackgroundColor3 = Color3.fromRGB(102,102,102)
	Text.TextScaled = true
	Text.BackgroundTransparency = 1
	MessageBox.Text = "" -- this a bit
	CharactersLeftValue = MaxCharactersValue
	CharactersLeftValue = 0
	MessageBox.TextEditable = true -- And this
	wait(5)
	MessageFrame:Remove()
	end
end)

Pretty much here, trying to make a server broadcast thing

FireServer automatically sends the player to the server, there's no need for manually doing that, basically what you have done now is sending the second argument on the server MessageBox, which gets a value from the client which is the player.

Also I wouldn't physically send the TextBox over the network, it'll only know how to transfer instances if both the Server and Client know of the instance existance. I'd rather just send the MessageBox.Text which is the text data of the textbox.

Edit: I would also like to point out how absolutely pointless this part is tostring(MessageBox.Text)

The text property is already of type string, there's no need to turn it into a string.