Convert http request for vb.net to http client

Question

I have this request in VB.NET (NET framework) that I need to convert to C# ASP.NET Core Http Client. The code in VB.NET looks like this:

Public Function PostRundown(data As String) As String
    Dim url = "https://url/import"

    Dim result As String = ""
    Dim httpRequest = CType(WebRequest.Create(url), HttpWebRequest)
    httpRequest.Method = "POST"
    httpRequest.ContentType = "application/x-www-form-urlencoded"

    'Dim authInfo = "xxx:xxx"
    Dim authInfo = config.NxtAuthInfo
    authInfo = Convert.ToBase64String(Encoding.Default.GetBytes(authInfo))
    httpRequest.Headers("Authorization") = "Basic " & authInfo
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls Or SecurityProtocolType.Tls11 Or SecurityProtocolType.Tls12 Or SecurityProtocolType.Ssl3
    ServicePointManager.ServerCertificateValidationCallback = AddressOf AcceptAllCertifications

    Using streamWriter = New StreamWriter(httpRequest.GetRequestStream())
        streamWriter.Write(data)
    End Using

    Dim httpResponse = CType(httpRequest.GetResponse(), HttpWebResponse)

    Using streamReader = New StreamReader(httpResponse.GetResponseStream())
        result = streamReader.ReadToEnd()
    End Using

    Return result
End Function

In C# I've started to form the code like below:

public async Task<string> CreateString(string url, string data)
{
    string authInfo = "xxx:xxx";
    authInfo = Convert.ToBase64String(Encoding.Default.GetBytes(authInfo));
    _client.DefaultRequestHeaders.Authorization =
    new AuthenticationHeaderValue("Authorization", "Basic " + authInfo);

    _client.DefaultRequestHeaders.Add("ContentType", "application/x-www-form-urlencoded");

    //var sjson = JsonConvert.SerializeObject(obj);
    var scontent = new StringContent(data, Encoding.UTF8, "application/x-www-form-urlencoded");
    //HttpResponseMessage response = await _client.PostAsJsonAsync<T>(url, obj);
    HttpResponseMessage response = await _client.PostAsync(url, scontent);

    if (response.StatusCode == System.Net.HttpStatusCode.OK)
        using (HttpContent content = response.Content)
        {
            var json = content.ReadAsStringAsync().Result;
            //var newObj = JsonConvert.DeserializeObject<T>(json);
            return json;
        }

    return "";
}

I'm not sure that the above code rightly represents the VB code. Also, I'm unsure how to implement the below lines in the HTTP client setting.

ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls Or SecurityProtocolType.Tls11 Or SecurityProtocolType.Tls12 Or SecurityProtocolType.Ssl3
ServicePointManager.ServerCertificateValidationCallback = AddressOf AcceptAllCertifications