The new chat system is copied to your PlayerScripts inside the player. So you want to require it from
game.Players.[PlayerName].PlayerScripts.ChatScript.ChatMain
That being said, if you don’t want to use those, you can just supply a table to ROBLOX’s end using the new SetCore option, like so:
game:GetService("StarterGui"):SetCore("CoreGuiChatConnections", {ChatWindow = {MessagePosted = BindableEvent}})
where the BindableEvent is what you’ll use to simulate the Chatted event.