luanti/games/devtest/mods/initial_message/init.lua

core.register_on_joinplayer(function(player)
	local cb = function(player)
		if not player or not player:is_player() then
			return
		end
		core.chat_send_player(player:get_player_name(), "This is the \"Development Test\" [devtest], meant only for testing and development.")
	end
	core.after(2.0, cb, player)
end)