How to create issue in jira using c#

I'm trying to create an issue in JIRA cloud using c# and the REST API, I get the 400 error (bad rquest), I dont know what I'm doing wrong !

This is My functions

    public static string PostJsonRequest(string endpoint, string userid, string password, string json)
    {
        // Create string to hold JSON response
        string jsonResponse = string.Empty;

        using (var client = new WebClient())
        {
            try
            {
                client.Encoding = System.Text.Encoding.UTF8;
                client.Headers.Set("Authorization", "Basic " + GetEncodedCredentials(userid, password));
                client.Headers.Add("Content-Type: application/json");
                client.Headers.Add("Accept", "application/json");
                var uri = new Uri(endpoint);
                var response = client.UploadString(uri, "POST", json);
                jsonResponse = response;
            }
            catch (WebException ex)
            {
                // Http Error
                if (ex.Status == WebExceptionStatus.ProtocolError)
                {
                    HttpWebResponse wrsp = (HttpWebResponse)ex.Response;
                    var statusCode = (int)wrsp.StatusCode;
                    var msg = wrsp.StatusDescription;
                    // throw new SmtpException(statusCode, msg);
                }
                else
                {
                    //  throw new HttpException(500, ex.Message);
                }
            }
        }

        return jsonResponse;
    }

And this is my JSON

string json = @"{
'fields':
{
    'project':
    {
        'key':'IS'
    },
    'summary':'REST Test',
    'issuetype':
    {
        'name':'Bug'
    },

}

}";

string Url = "https://XXXXXX.atlassian.net/rest/api/2/issue/";

Any Ideas on what I'm doing wrong ?