How can I make the return type be Task?

Question

C++ / C#

Trying to apply the following solutions for websockets ( https://github.com/jjrdk/websocket-sharp/blob/mast... and immediately ran into a problem.
Code:

public class Echo : WebSocketBehavior
  {
    protected override void OnMessage(MessageEventArgs e)
    { Send(e.Data); }
  }

в браузере сборок метод OnMessage выглядит так:

using System;
using System.Threading.Tasks;

protected virtual Task OnMessage (MessageEventArgs e);

An error appears on the OnMessage method, the content of which is:

Error CS0508: 'Echo.OnMessage(MessageEventArgs)': return type must be 'Task' to match member overridden 'WebSocketBehavior.OnMessage(MessageEventArgs)'.

How to solve this problem?