You've not specified what you're writing it in, so I'm going to describe the simplest way you can do this, using the InSim protocol. You'll need to adapt it for whatever library you're using.
On IS_NCN (InSim NewConnectioN) send one or more IS_BTN (Insim BuTtoN) to the ucid of the new connection Set the IS_BTN properties to match what you want to display (i.e. text = "welcome, blah blah". If you want to autoremove it after X seconds, send a IS_BFN (InSim Button FunctioN) with a subtype of BFN_CLEAR (clears all buttons created by this InSim instance on this user's screen).
If you want the user to click to remove, listen for a IS_BTC (InSim BuTton Click) with the clickid set to your close button's clickid, then send a IS_BFN with a subtype of BFN_CLEAR.