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You would want to use the SetCore function of StarterGui via a LocalScript and use the parameter "ChatMakeSystemMessage"

Here is the example from the Roblox wiki:

game:GetService("StarterGui"):SetCore("ChatMakeSystemMessage",{
	Text = "Welcome to my game!"; -- Required. Has to be a string!
	Color = Color3.new(0,1,1); -- Cyan is (0,255/255,255/255) -- Optional defaults to white: Color3.new(255/255, 255/255, 243/255)
	Font = Enum.Font.SourceSans; -- Optional, defaults to Enum.Font.SourceSansBold
	FontSize = Enum.FontSize.Size24; -- Optional, defaults to Enum.FontSize.Size18
})

As of this time it does not look like it follows Enumeration but using a string will work. Some functions will not work with SetCore yet either and it must be run in a LocalScript. There are different values required for CharMakeSystemMessage. Those are Text (to make the chat text), Color (Color3 for different styles of color), Font (For different fonts), and FontSize (For different sizes of that font).