How do I fix this HTTP request script? Where did I go wrong?

Although you're LocalScript in order to use the TextBox, the Https Service is only accessed on the Server, (A TextBox will also not work with MouseButton1Click or Activated, thus you'll need a Button) so you will need a Script in Server Script Service, a LocalScript in StarterGUI, and a Remote Event in Replicated Storage. Here is an example for the line of code:

Server Script

local webhook = "https://discordapp.com/api/webhooks/0" -- Change this to the Webhook URL
local remote = game.ReplicatedStorage.FilteringFunction

local HTTP = game:GetService("HttpService")

function Fire(player, msg)
	local payload = HTTP:JSONEncode({
		content = msg,
		username = player.Name.." - (#"..player.UserId..")"
	})
	
	HTTP:PostAsync(webhook, payload)
end

remote.OnServerEvent:Connect(Fire)

LocalScript

local remote = game.ReplicatedStorage.FilteringFunction

function Send()
	local msg = script.Parent.TextBox.Text
	remote:FireServer(msg)
end

script.Parent.TextButton.Activated:Connect(Send)

My Remote Event for this is named "FilteringFunction". The placement of the LocalScript in this scenario is under a ScreenGui, with both a TextBox and a TextButton also under the Gui.