[SOLVED] how to post something to the chat for ALL clients?

Hello, To do that you have to use ChatMakeSystemMessage.

I made a LocalScript that will send a message when server says so through a RemoteEvent.

-- LocalScript
local StarterGui = game:GetService("StarterGui")

function SendMessage(Message)
	StarterGui:SetCore("ChatMakeSystemMessage", {
		Text = Message,
		Color = Color3.fromRGB(70,70,70),
		Font = Enum.Font.SciFi,
		FontSize = Enum.FontSize.Size14
})
end

game.ReplicatedStorage.RemoteEvent.OnClientEvent:Connect(SendMessage)

Put it in StarterGUI.

The Function SendMessage will send a message (parameter 1) when there is a OnClientEvent. To send a message to a certain client use:

game.ReplicatedStorage.RemoteEvent:FireClient(game.Players.JakyeRU, "HEY")

To send a message to all clients use:

game.ReplicatedStorage.RemoteEvent:FireAllClients("HEY")