You can use StarterGui:SetCore to do this.
Here’s an example:
local starterGui = game:GetService("StarterGui")
local msg = "Test"
starterGui:SetCore("ChatMakeSystemMessage",{Text = msg, Font = Enum.Font.Legacy, Color = Color3.fromRGB(255,0,0)})
Using the example @Lit_skillzYT provided, you can format the message sent by skipping lines using "\n"
