In the RustPlugin I could use:

        private void Message(BasePlayer player, string message)
        {
            Player.Message(player, message, id);
        }​

and it worked and the icon was of a given id.

In CovalencePlugin I tried:

private void TestCommand(IPlayer player, string command, string[] args)
        {
            object[] ss = new object[1];
            ss[0] = 76561198091434721;
            player.Reply(command, "test", ss);
			
	object[] ss2 = new object[2];
	ss2[0] = null;
	ss2[1] = 76561198091434721;
        player.Reply("test", "test2", ss2);
	player.Message("test", "pre", null, "76561198091434721");
        }

but none of this work.

How can I achieve chat avatar in Covalence?

There are no universal methods for that yet, so you'd have to use the method you were using before else call a Rust method directly.

https://github.com/OxideMod/Oxide.Rust/blob/develop/src/Libraries/Player.cs#L367-L369

Thanks,

quick paste for lazy ppl:

private void TestCommand(IPlayer player)
        {
            var basePlayer = player.Object as BasePlayer;
            string message = "msg";
            string prefix = "pre";
            var userId = 76561198091434721;
            string formatted = prefix != null ? $"{prefix} {message}" : message;
            basePlayer.SendConsoleCommand("chat.add", 2, userId, formatted);
        }​

        private void TestCommand(IPlayer player)
        {
            string text= $"{prefix} {message}";
            (player.Object as BasePlayer).SendConsoleCommand("chat.add", 2, 76561198091434721, text);
        }

Something a little more condensed.