public void PostToSlack(string text, string username = null, string channel = null)
    {
        Payload payload = new Payload()
        {
            Channel = channel,
            Username = username,
            Text = text
        };

        string payloadJson = JsonConvert.SerializeObject(payload);

        using (WebClient client = new WebClient())
        {
            NameValueCollection data = new NameValueCollection();
            data["payload"] = payloadJson;

            var response = client.UploadValues("your slack webhook url here", "POST", data);
        }
    }

public class Payload
{
    [JsonProperty("channel")]
    public string Channel { get; set; }

    [JsonProperty("username")]
    public string Username { get; set; }

    [JsonProperty("text")]
    public string Text { get; set; }
}

And info on how to create the webhook url: http://www.programmableweb.com/news/how-to-integrate-webhooks-slack-api/how-to/2015/10/20