local TextChatService = game:GetService("TextChatService")
local Players = game:GetService("Players")
local LocalPlayer = Players.LocalPlayer
local Tag = {
    Color = "#6df76f", -- you can change it to what ever you like, BUT IT HAS TO BE HEX
    Chattag = "[Lyxx User]" -- this is what your tag will be
}

TextChatService.OnIncomingMessage = function(Message, ChatStyle)
    local MessageProperties = Instance.new("TextChatMessageProperties")
    local Player = Players:GetPlayerByUserId(Message.TextSource.UserId)
    if Player.Name == LocalPlayer.Name then
        MessageProperties.PrefixText = '<font color="' .. Tag.Color .. '">' .. Tag.Chattag .. '</font> ' .. Message.PrefixText
    end
    return MessageProperties
end