How do people custom the RobloxGui Chat?

Hello. I've been going to many games recently, and some games have had the RobloxGui Chat GUI, but they have special things in them, such as they say " has joined the Server!", hints, etc.

I've tried many things to figure out how they do this, such as: game.CoreGui.RobloxGui.Parent = game.Workspace or game.CoreGui.RobloxGui.Parent = game.StarterGui etc., I would then edit the script, such as add my group to the adminCashe, etc., but it never works.

How do they do this? Please reply if you know. It'd help a lot.

local Siggy = Instance.new("TextLabel") .. Siggy.Text = "~Thanks, SkidamrkLive!"